At all times there can only be a max of 10 enderman in a given dimension/around a player, the spawn time for the 10 to spawn is also really bad, from what I have seen with a simple enderman farm it can take up to 2 minutes to fully spawn 10 enderman, however, this can also be contributed to the fact that its really about a 30% chance the the enderman decides to target an endermite within the first 4 seconds after spawning even though the endermite is only 8 blocks away from it.

Unfortunately, no one was smart enough to add a screenshot button to this version of the game so it would be impossible for me to provide one.