So we own server on site Hicoria and we wanna build a enderman xp farm but everywhere in there is like no endermen at all.. Theyre spawning in groups of 4 and really rarely.. difficulty is normal and it was hard for a test and it didn't help..
Mobs spawn more at lower Y levels - try building a platform around y=1
You'd only ever even notice this when building a mob farm which tries to maximize spawn rates by keeping the mob count below the cap, otherwise, the mob cap is for all intents and purposes the only factor limiting spawn rates, even if the actual number of attempts at spawning mobs was greatly reduced - I've never had issues with mobs not spawning or not spawning enough even in a "triple height terrain" world, with sea level at y=191, or in my current mod, which reduces per-chunk spawn rates by a factor of 4 for the equivalent of 4x the normal sea level, yet while caving I encounter about double the mobs as in vanilla due to a reduced spawn range increasing the density; even with both of these factors (only around 30 chunks able to spawn mobs per tick, compared to 225 in vanilla) the mob count only takes 5-10 seconds to reach the cap after switching from Peaceful, which is enough to keep up with normal player movement.
I don't think it applies outside of the 1.16 Nether, but they added a new layer to the spawning system for mobs that appear in the Nether. This layer prevents too many mobs of the same type from spawning in close proximity to each other, and the areas of effect around each mob individual combine into a larger centroid.
Endermen are one of the mobs that has this layer, but like I said I don't think it applies to the endermen spawning in either the overworld or the End...but then again in the 1.13 snapshots Mojang DID also accidentally allow dryland hostile mobs to spawn in the water.