4,194,304 to be exact, which repeat at various powers of 2 along one axis:

This is also just for all possible 48 bit seeds - leading to a total of nearly 275 billion seeds which repeat within the bounds of a Minecraft world (60 million blocks, or 3.75 million chunks, across, which is why the highest repeat interval is 2097152 chunks - in theory every single seed will eventually repeat after 2^48 chunks at the most):

Here are some examples of various repeat intervals (mineshafts are not shown since I used a self-written program instead of the actual game to generate them much faster than the game/Unmined can; the caves/ravines match those in 1.6.4 but all versions from Beta 1.8 to 1.13 are affected in the same manner):

The classic "107038380838084" seed, which repeats every chunk:

"246345201500483", which repeats every 2 chunks:

"50589139596712", which repeats every 4 chunks:

"218093180616145", which repeats every 8 chunks:

"5720409445043", which repeats every 16 chunks:

"254092702608805", which repeats every 32 chunks (all of these images are 64x64 chunks):

Also, it is interesting to note that this bug also affects other structures; for example, this is what happens when you use the seed "107038380838084" in a Superflat world with village(distance=9), which removes the normal offset that they have, but this is not necessary as they will be the same as long as their z coordinates are the same:

The reason why they do not seem to be affected otherwise is because they use a different method of setting the chunk seed when calculating their offsets relative to the grid they generate at (32x32 chunks by default with an offset of 0-23, meaning that any two villages have a 1/24 chance of having the same x or z coordinates), but they do use the same method as caves when setting the seed which determines their layout. All minor features, such as ores, are not affected at all since they use yet another method of setting the chunk seed which forces the multipliers to always be odd numbers, but it does mean that there are only half as many unique values.

There is also another bug, which I call the "mirrored caves bug", which I discovered myself and have mentioned before which causes chunks at sign-reversed coordinate pairs to have the same chunk seed, with as many as 1/3 of all chunks in a given seed affected (other seeds have 1/6, 1/12, 1/24, etc), and which affects far more seeds; like the previous bug it mainly affects caves and mineshafts but it can also result in identical structures if their random offsets happen to place them at the right coordinates.

Here are some maps of the "mirrored caves" bug, with only caves in affected chunks shown and all caves:

"18", with 1/3 of chunks affected:

"10", with 1/6 of chunks affected:

"1", with 1/12 of chunks affected:

As seen in this list of seeds from 0-99 practically every seed is affected to varying degrees; I'd consider the seeds with 1/3 of chunks affected as being affected enough to impact their usability for somebody like myself unless you restrict yourself to one side of the x or z axis (due to the fact that individual caves are not flipped around and are placed differently relative to other caves, even ones that are also mirrored, it is much less apparent outside of larger caves):

Seed: 0; percentage of chunks that match is 8.30227%
Seed: 1; percentage of chunks that match is 8.30227%
Seed: 2; percentage of chunks that match is 4.152845%
Seed: 3; percentage of chunks that match is 8.30227%
Seed: 4; percentage of chunks that match is 4.152845%
Seed: 5; percentage of chunks that match is 8.30227%
Seed: 6; percentage of chunks that match is 4.152845%
Seed: 7; percentage of chunks that match is 4.152845%
Seed: 8; percentage of chunks that match is 4.152845%
Seed: 9; percentage of chunks that match is 4.152845%
Seed: 10; percentage of chunks that match is 16.60264%
Seed: 11; percentage of chunks that match is 4.152845%
Seed: 12; percentage of chunks that match is 8.30227%
Seed: 13; percentage of chunks that match is 4.152845%
Seed: 14; percentage of chunks that match is 8.30227%
Seed: 15; percentage of chunks that match is 8.30227%
Seed: 16; percentage of chunks that match is 8.30227%
Seed: 17; percentage of chunks that match is 4.152845%
Seed: 18; percentage of chunks that match is 33.2049%
Seed: 19; percentage of chunks that match is 2.0796523%
Seed: 20; percentage of chunks that match is 33.2049%
Seed: 21; percentage of chunks that match is 16.60264%
Seed: 22; percentage of chunks that match is 33.2049%
Seed: 23; percentage of chunks that match is 33.2049%
Seed: 24; percentage of chunks that match is 4.152845%
Seed: 25; percentage of chunks that match is 4.152845%
Seed: 26; percentage of chunks that match is 16.60264%
Seed: 27; percentage of chunks that match is 33.2049%
Seed: 28; percentage of chunks that match is 16.60264%
Seed: 29; percentage of chunks that match is 4.152845%
Seed: 30; percentage of chunks that match is 33.2049%
Seed: 31; percentage of chunks that match is 16.60264%
Seed: 32; percentage of chunks that match is 16.60264%
Seed: 33; percentage of chunks that match is 16.60264%
Seed: 34; percentage of chunks that match is 33.2049%
Seed: 35; percentage of chunks that match is 33.2049%
Seed: 36; percentage of chunks that match is 33.2049%
Seed: 37; percentage of chunks that match is 8.30227%
Seed: 38; percentage of chunks that match is 33.2049%
Seed: 39; percentage of chunks that match is 33.2049%
Seed: 40; percentage of chunks that match is 33.2049%
Seed: 41; percentage of chunks that match is 33.2049%
Seed: 42; percentage of chunks that match is 4.152845%
Seed: 43; percentage of chunks that match is 8.30227%
Seed: 44; percentage of chunks that match is 33.2049%
Seed: 45; percentage of chunks that match is 16.60264%
Seed: 46; percentage of chunks that match is 33.2049%
Seed: 47; percentage of chunks that match is 33.2049%
Seed: 48; percentage of chunks that match is 33.2049%
Seed: 49; percentage of chunks that match is 8.30227%
Seed: 50; percentage of chunks that match is 4.152845%
Seed: 51; percentage of chunks that match is 16.60264%
Seed: 52; percentage of chunks that match is 33.2049%
Seed: 53; percentage of chunks that match is 16.60264%
Seed: 54; percentage of chunks that match is 33.2049%
Seed: 55; percentage of chunks that match is 33.2049%
Seed: 56; percentage of chunks that match is 33.2049%
Seed: 57; percentage of chunks that match is 33.2049%
Seed: 58; percentage of chunks that match is 16.60264%
Seed: 59; percentage of chunks that match is 16.60264%
Seed: 60; percentage of chunks that match is 8.30227%
Seed: 61; percentage of chunks that match is 33.2049%
Seed: 62; percentage of chunks that match is 8.30227%
Seed: 63; percentage of chunks that match is 2.0796523%
Seed: 64; percentage of chunks that match is 33.2049%
Seed: 65; percentage of chunks that match is 33.2049%
Seed: 66; percentage of chunks that match is 16.60264%
Seed: 67; percentage of chunks that match is 16.60264%
Seed: 68; percentage of chunks that match is 33.2049%
Seed: 69; percentage of chunks that match is 4.152845%
Seed: 70; percentage of chunks that match is 16.60264%
Seed: 71; percentage of chunks that match is 33.2049%
Seed: 72; percentage of chunks that match is 33.2049%
Seed: 73; percentage of chunks that match is 33.2049%
Seed: 74; percentage of chunks that match is 8.30227%
Seed: 75; percentage of chunks that match is 2.0796523%
Seed: 76; percentage of chunks that match is 16.60264%
Seed: 77; percentage of chunks that match is 33.2049%
Seed: 78; percentage of chunks that match is 33.2049%
Seed: 79; percentage of chunks that match is 16.60264%
Seed: 80; percentage of chunks that match is 16.60264%
Seed: 81; percentage of chunks that match is 1.0460958%
Seed: 82; percentage of chunks that match is 16.60264%
Seed: 83; percentage of chunks that match is 16.60264%
Seed: 84; percentage of chunks that match is 16.60264%
Seed: 85; percentage of chunks that match is 33.2049%
Seed: 86; percentage of chunks that match is 16.60264%
Seed: 87; percentage of chunks that match is 16.60264%
Seed: 88; percentage of chunks that match is 16.60264%
Seed: 89; percentage of chunks that match is 16.60264%
Seed: 90; percentage of chunks that match is 33.2049%
Seed: 91; percentage of chunks that match is 33.2049%
Seed: 92; percentage of chunks that match is 8.30227%
Seed: 93; percentage of chunks that match is 16.60264%
Seed: 94; percentage of chunks that match is 16.60264%
Seed: 95; percentage of chunks that match is 16.60264%
Seed: 96; percentage of chunks that match is 16.60264%
Seed: 97; percentage of chunks that match is 16.60264%
Seed: 98; percentage of chunks that match is 8.30227%
Seed: 99; percentage of chunks that match is 8.30227%

In fact, about a third of all seeds from 0-9999, which should be representative of all seeds, have 1/3 of chunks matching, with another third having 1/6, and only 0.23% were completely unaffected, at least within 256 chunks of the origin, the area I checked (much like the first bug this bug has an "interval" which increases in powers of 2 so larger areas will reveal more seeds):

Percentage of seeds with 1/3 of chunks matching: 33.37%
Percentage of seeds with 1/6 of chunks matching: 33.37%
Percentage of seeds with 1/12 of chunks matching: 16.5%
Percentage of seeds with 1/24 of chunks matching: 8.24%
Percentage of seeds with 1/48 of chunks matching: 4.24%
Percentage of seeds with 1/96 of chunks matching: 2.14%
Percentage of seeds with less than 1/96 of chunks matching: 1.91%
Percentage of seeds with no chunks matching: 0.23%

Of interest, the seed for my first world, "-123775873255737467", has 1/6 of chunks affected and I've recognized that I explored the same caves twice (as mentioned above, the different relative positions and overlap between caves makes this generally unnoticeable, except when it affects larger caves).

All of this gives me the impression that Mojang should switch to a better RNG (64 bit, Java's Random is also quite slow) and/or method of calculating the chunk seed, especially if they make a major update to the underground, which could make these effects much more apparent, especially the "mirrored caves" one, which was quite apparent in older versions of my mods due to the much larger caves/ravines they have (I've been able to fix all of the issues mentioned here by using my own 64 bit RNG and chunk seed setting method which appears to not have any of these issues, though it can be hard to test and any of the seeds given above are not applicable, but if the x/z "modifiers" are 0 they don't zero out the effects of their corresponding axis since they are not multipliers, and XOR is not used to combine the x/z components, which is what leads to the second issue; if any seeds do indeed break it they would also break the game's biome generator since I based it off of its chunk seed method and so far no such seeds have been found).

You've probably heard about the seed "107038380838084", which causes caves and some other features to repeat indefinitely along the x-axis, or its counterpart, "164311266871034", which does the same for the z-axis - but there are actually many more seeds which repeat to some degree - and not just seeds which only differ in the uppermost 16 bits, which have no effect on non-biome dependent features (e.g. ores, while villages will only match where the biome matches).

4,194,304 to be exact, which repeat at various powers of 2 along one axis:

This is also just for all possible 48 bit seeds - leading to a total of nearly 275 billion seeds which repeat within the bounds of a Minecraft world (60 million blocks, or 3.75 million chunks, across, which is why the highest repeat interval is 2097152 chunks - in theory every single seed will eventually repeat after 2^48 chunks at the most):

Here are some examples of various repeat intervals (mineshafts are not shown since I used a self-written program instead of the actual game to generate them much faster than the game/Unmined can; the caves/ravines match those in 1.6.4 but all versions from Beta 1.8 to 1.13 are affected in the same manner):

The classic "107038380838084" seed, which repeats every chunk:

"246345201500483", which repeats every 2 chunks:

"50589139596712", which repeats every 4 chunks:

"218093180616145", which repeats every 8 chunks:

"5720409445043", which repeats every 16 chunks:

"254092702608805", which repeats every 32 chunks (all of these images are 64x64 chunks):

Also, it is interesting to note that this bug also affects other structures; for example, this is what happens when you use the seed "107038380838084" in a Superflat world with village(distance=9), which removes the normal offset that they have, but this is not necessary as they will be the same as long as their z coordinates are the same:

The reason why they do not seem to be affected otherwise is because they use a different method of setting the chunk seed when calculating their offsets relative to the grid they generate at (32x32 chunks by default with an offset of 0-23, meaning that any two villages have a 1/24 chance of having the same x or z coordinates), but they do use the same method as caves when setting the seed which determines their layout. All minor features, such as ores, are not affected at all since they use yet another method of setting the chunk seed which forces the multipliers to always be odd numbers, but it does mean that there are only half as many unique values.

There is also another bug, which I call the "mirrored caves bug", which I discovered myself and have mentioned before which causes chunks at sign-reversed coordinate pairs to have the same chunk seed, with as many as 1/3 of all chunks in a given seed affected (other seeds have 1/6, 1/12, 1/24, etc), and which affects far more seeds; like the previous bug it mainly affects caves and mineshafts but it can also result in identical structures if their random offsets happen to place them at the right coordinates.

Here are some maps of the "mirrored caves" bug, with only caves in affected chunks shown and all caves:

"18", with 1/3 of chunks affected:

"10", with 1/6 of chunks affected:

"1", with 1/12 of chunks affected:

As seen in this list of seeds from 0-99 practically every seed is affected to varying degrees; I'd consider the seeds with 1/3 of chunks affected as being affected enough to impact their usability for somebody like myself unless you restrict yourself to one side of the x or z axis (due to the fact that individual caves are not flipped around and are placed differently relative to other caves, even ones that are also mirrored, it is much less apparent outside of larger caves):

Seed: 1; percentage of chunks that match is 8.30227%

Seed: 2; percentage of chunks that match is 4.152845%

Seed: 3; percentage of chunks that match is 8.30227%

Seed: 4; percentage of chunks that match is 4.152845%

Seed: 5; percentage of chunks that match is 8.30227%

Seed: 6; percentage of chunks that match is 4.152845%

Seed: 7; percentage of chunks that match is 4.152845%

Seed: 8; percentage of chunks that match is 4.152845%

Seed: 9; percentage of chunks that match is 4.152845%

Seed: 10; percentage of chunks that match is 16.60264%

Seed: 11; percentage of chunks that match is 4.152845%

Seed: 12; percentage of chunks that match is 8.30227%

Seed: 13; percentage of chunks that match is 4.152845%

Seed: 14; percentage of chunks that match is 8.30227%

Seed: 15; percentage of chunks that match is 8.30227%

Seed: 16; percentage of chunks that match is 8.30227%

Seed: 17; percentage of chunks that match is 4.152845%

Seed: 18; percentage of chunks that match is 33.2049%

Seed: 19; percentage of chunks that match is 2.0796523%

Seed: 20; percentage of chunks that match is 33.2049%

Seed: 21; percentage of chunks that match is 16.60264%

Seed: 22; percentage of chunks that match is 33.2049%

Seed: 23; percentage of chunks that match is 33.2049%

Seed: 24; percentage of chunks that match is 4.152845%

Seed: 25; percentage of chunks that match is 4.152845%

Seed: 26; percentage of chunks that match is 16.60264%

Seed: 27; percentage of chunks that match is 33.2049%

Seed: 28; percentage of chunks that match is 16.60264%

Seed: 29; percentage of chunks that match is 4.152845%

Seed: 30; percentage of chunks that match is 33.2049%

Seed: 31; percentage of chunks that match is 16.60264%

Seed: 32; percentage of chunks that match is 16.60264%

Seed: 33; percentage of chunks that match is 16.60264%

Seed: 34; percentage of chunks that match is 33.2049%

Seed: 35; percentage of chunks that match is 33.2049%

Seed: 36; percentage of chunks that match is 33.2049%

Seed: 37; percentage of chunks that match is 8.30227%

Seed: 38; percentage of chunks that match is 33.2049%

Seed: 39; percentage of chunks that match is 33.2049%

Seed: 40; percentage of chunks that match is 33.2049%

Seed: 41; percentage of chunks that match is 33.2049%

Seed: 42; percentage of chunks that match is 4.152845%

Seed: 43; percentage of chunks that match is 8.30227%

Seed: 44; percentage of chunks that match is 33.2049%

Seed: 45; percentage of chunks that match is 16.60264%

Seed: 46; percentage of chunks that match is 33.2049%

Seed: 47; percentage of chunks that match is 33.2049%

Seed: 48; percentage of chunks that match is 33.2049%

Seed: 49; percentage of chunks that match is 8.30227%

Seed: 50; percentage of chunks that match is 4.152845%

Seed: 51; percentage of chunks that match is 16.60264%

Seed: 52; percentage of chunks that match is 33.2049%

Seed: 53; percentage of chunks that match is 16.60264%

Seed: 54; percentage of chunks that match is 33.2049%

Seed: 55; percentage of chunks that match is 33.2049%

Seed: 56; percentage of chunks that match is 33.2049%

Seed: 57; percentage of chunks that match is 33.2049%

Seed: 58; percentage of chunks that match is 16.60264%

Seed: 59; percentage of chunks that match is 16.60264%

Seed: 60; percentage of chunks that match is 8.30227%

Seed: 61; percentage of chunks that match is 33.2049%

Seed: 62; percentage of chunks that match is 8.30227%

Seed: 63; percentage of chunks that match is 2.0796523%

Seed: 64; percentage of chunks that match is 33.2049%

Seed: 65; percentage of chunks that match is 33.2049%

Seed: 66; percentage of chunks that match is 16.60264%

Seed: 67; percentage of chunks that match is 16.60264%

Seed: 68; percentage of chunks that match is 33.2049%

Seed: 69; percentage of chunks that match is 4.152845%

Seed: 70; percentage of chunks that match is 16.60264%

Seed: 71; percentage of chunks that match is 33.2049%

Seed: 72; percentage of chunks that match is 33.2049%

Seed: 73; percentage of chunks that match is 33.2049%

Seed: 74; percentage of chunks that match is 8.30227%

Seed: 75; percentage of chunks that match is 2.0796523%

Seed: 76; percentage of chunks that match is 16.60264%

Seed: 77; percentage of chunks that match is 33.2049%

Seed: 78; percentage of chunks that match is 33.2049%

Seed: 79; percentage of chunks that match is 16.60264%

Seed: 80; percentage of chunks that match is 16.60264%

Seed: 81; percentage of chunks that match is 1.0460958%

Seed: 82; percentage of chunks that match is 16.60264%

Seed: 83; percentage of chunks that match is 16.60264%

Seed: 84; percentage of chunks that match is 16.60264%

Seed: 85; percentage of chunks that match is 33.2049%

Seed: 86; percentage of chunks that match is 16.60264%

Seed: 87; percentage of chunks that match is 16.60264%

Seed: 88; percentage of chunks that match is 16.60264%

Seed: 89; percentage of chunks that match is 16.60264%

Seed: 90; percentage of chunks that match is 33.2049%

Seed: 91; percentage of chunks that match is 33.2049%

Seed: 92; percentage of chunks that match is 8.30227%

Seed: 93; percentage of chunks that match is 16.60264%

Seed: 94; percentage of chunks that match is 16.60264%

Seed: 95; percentage of chunks that match is 16.60264%

Seed: 96; percentage of chunks that match is 16.60264%

Seed: 97; percentage of chunks that match is 16.60264%

Seed: 98; percentage of chunks that match is 8.30227%

Seed: 99; percentage of chunks that match is 8.30227%

In fact, about a third of all seeds from 0-9999, which should be representative of all seeds, have 1/3 of chunks matching, with another third having 1/6, and only 0.23% were completely unaffected, at least within 256 chunks of the origin, the area I checked (much like the first bug this bug has an "interval" which increases in powers of 2 so larger areas will reveal more seeds):

Of interest, the seed for my first world, "-123775873255737467", has 1/6 of chunks affected and I've recognized that I explored the same caves twice (as mentioned above, the different relative positions and overlap between caves makes this generally unnoticeable, except when it affects larger caves).

All of this gives me the impression that Mojang should switch to a better RNG (64 bit, Java's Random is also quite slow) and/or method of calculating the chunk seed, especially if they make a major update to the underground, which could make these effects much more apparent, especially the "mirrored caves" one, which was quite apparent in older versions of my mods due to the much larger caves/ravines they have (I've been able to fix all of the issues mentioned here by using my own 64 bit RNG and chunk seed setting method which appears to not have any of these issues, though it can be hard to test and any of the seeds given above are not applicable, but if the x/z "modifiers" are 0 they don't zero out the effects of their corresponding axis since they are not multipliers, and XOR is not used to combine the x/z components, which is what leads to the second issue; if any seeds do indeed break it they would also break the game's biome generator since I based it off of its chunk seed method and so far no such seeds have been found).

TheMasterCaver's First World - possibly the most caved-out world in Minecraft history - includes world download.

TheMasterCaver's World - my own version of Minecraft largely based on my views of how the game should have evolved since 1.6.4.

Why do I still play in 1.6.4?