The limit is per loaded chunk, so players in the same place have the same limit as a single player, spread out players have more mobs the more players there are.
But you don't get your own private limit, the limit is applied to all of the loaded chunks.
When you kill endermen new ones spawn but they can spawn anywhere they are able to.
So unless they are killed anywhere they spawn, the places where they don't get killed will fill up and the places they are killed get fewer and fewer spawning.
Why other farms work better than yours I don't know, maybe they are bigger (more likely to spawn endermen) or maybe they are being used when fewer players are in the end (or fewer players that aren't killing all the endermen that spawn around them anyway).