I assume there are some people who have written FPS in Java here. Is it useful to display more than 10 enemies in the field of view in a FPS? I think that when there are 10 enemies in front of you, the screen is full or you don't pay attention to the 10 enemies at the same time. Therefore I wonder whether it is important to optimize an engine to be able to do this and it depends on the complexity of the models.
However, I think it might be interesting when you're in an aircraft or an helicopter and you are shooting guys on the floor.
I can't coin having finished a fps in java but I have played (team) fps on a pretty decent level - tracking 10 ppl who aren't even on your screen is typical (extrapolate their possible paths ~run speed from where they where last seen). As far as enemy on the screen go one can track quite a lot more, even ppl who are not blessed with that ability can still follow clusters/'streams' of enemies, so even while they can't track them individually you need to see them all. Now even if you question those abilities(I believe some studies said they proved that ppl can't track more then 3 things at once.) There is still the inability to determine which enemy the player is interested. Or rather the other way around: which one can you safely remove?
Stuff you can't see you don't need to render, so if they truly fill the screen: you are not going to render them to begin with. And with that it could just as well be just one enemy right in your face. 10 is not a magic number.
Reducing detail when stuff is further away (unreal engine among others does this from early on(it uses 2 radius's, it's pretty easy to spot in UT99 and you can also configure it I think in later versions they made it dynamic)) has little to do with a humans ability to track stuff. has to do with resolution and angles - you can actually prove it mathematically.
btw: seeing your allies is just as important with friendly fire on