The mob cap for endermen is (70 - the number of other hostile mobs currently in existence).
Note that in 1.16 endermen are one of the mobs that use the new spawning zone mechanics, which prevents other mobs of the same type from spawning in a small area around them. These areas meld together to form one bigger area encompassing all members.
Having many mobs within a small area, close enough so they collide, especially within a single block, can cause server lag due to the collision calculations (Spigot even has a setting, max-entity-collisions, which limits how many entities an entity can collide with to reduce lag) but aside from rendering there won't be any direct effect on client-side performance unless you have a very old CPU with only 1-2 cores, but they aren't suitable for running the game in any case. This is also likely why Mojang added entity cramming, which effectively limits collisions and lag by killing them. It should also noted that the server only sends entities within 80 blocks of a player to the client, so with normal mob spawning a significant percentage of mobs will be further away (because of this, F3, which only shows client-side entities, is not a reliable way to see if you are below the mob cap).