I've mined 2,157 emerald ore in my first world, which represents about half my total playtime, though the amount of emerald that I've mined in total is probably less than twice that since many of my other worlds have less Extreme Hills and/or my own modded biomes which contain emeralds (I found none at all in a few worlds).
To put things into perspective, I've mined a total of 2.7 million ore in my first world, including 14,270 diamond ore, 660,000 iron ore, and 1.8 million coal ore; this means that emerald is about 0.08% of all the ores I've mined; with the equivalent of around 900 play sessions spent caving this is an average of about 2.4 emerald out of 3,000 ores mined per session (of course, the actual amount varies wildly; I can find considerably more emerald than diamond under an Extreme Hills).
For comparison, in TMCWv4, one of my modded worlds with drastically different world generation from vanilla (meaning 1.6.4, where Extreme Hills was 1/7 of biomes outside of Ice Plains regions and oceans), I found 145 emerald ore and a total of 683,979 ore, making emerald only 0.021% of the total; I averaged only 0.65 emerald out of 3,080 ore mined per play session spent caving, with hourly rates being lower (I averaged about 3.5 hours per session in my first world and 3.83 hours in TMCWv4 for rates of 0.685 and 0.17 emerald ore and 857 and 803 total ore per hour respectively).
Conversely, in TMCWv1 I found considerably more emerald, 947 out of a total of 773,152 ore, or 0.122% of the total, making it about 1.5 times more common than in my first world (this was an earlier version of TMCW with far fewer biomes, and more of the biomes I explored were Extreme Hills or the equivalent, including two of my own "volcanic wasteland" biomes, which have more of all ores, especially emerald, which generates in multi-block veins).
Aside from mining, I've traded for more than 8,000 emeralds in my first world, which I trade in order to buy diamond gear to repair my gear (this is not necessary given the number of diamonds I've collected - an entire chest of diamond blocks); I traded about 800 emeralds in TMCWv4 in order to obtain Mending (this is not 1.9 but my own version, with 1.6.4 trading mechanics). These emeralds are kept entirely separate from the emeralds I find while caving, which I mine with Silk Touch (I used Fortune earlier on, with a couple stacks of emerald blocks from this in my first world), plus the occasional emeralds from loot chests (in my first world I just add these to my trading stockpile). Otherwise, I haven't traded in other worlds so this represents pretty much all the emeralds I've obtained from trading (most of which have been traded for other items so I don't actually have 8,000+ emeralds laying around, just a few stacks).