Mob cap for a single block is 24 mobs. After that they start dying off. Without any modifications you would be able to fit 192 mobs in a 4x2 area (8 blocks). You can do things like adding ladders onto the side to kind of get around the mob cap because some mobs will go up and down the ladder making it so they are not always in the block.
maxEntityCramming is not a per-block limit; it is the number of entities that any one entity can collide with, meaning that if they are smaller than a block you can fit more in; for example, zombies are 0.6 blocks wide, meaning that they have to be at least that far apart (center-center) in order to not collide:
The maximum number of other pushable entities a mob or player can push, before taking 3 suffocation damage per half-second.
Given this, it can be calculated that 432 zombies could fit in a 2x4 space before they start suffocating (their area of 0.36 blocks would give about 533 but you can only fit 3x6 zombies in a 2x4 space). The best way to find out an exact answer would be to do actual tests (spawn mobs in Creative until they begin to die) since you have to account for them colliding and pushing each other around and the real answer is probably a lot lower, even as low as 24 for guaranteed survival of all mobs, and due to variable collision rates the death rate will gradually increase instead of suddenly hitting a threshold (if you are using this to e.g. collect mobs in a kill chamber this will not matter as long as they accumulate much faster than they die).