Yep, I agree, it should certainly depend on the chosen gamemode.
Very huge maps should definitaly have 64 players for gigantic battles.
Well, remembering Dark Ages of Camelot, it was really cool attacking/defending a castle with many, many, many, many other players
But that would require an own game mode for maximum fun.
DeathMatch: To be honest I don`t like this game mode that much (simply lacks team tactics), I think
16 players should be more than enough
TeamDeathMatch: Due it`s been played on the same map as DM.... ->
16 players
Capture the flag: A difficult choice, due it makes also fun with many bots, but I think
16 players should do.
Onslaught: In case the maps won`t get even more bigger and with more power nodes ->
32 players
Conquest: Don`t now enough to say something to that *g*
A pitty, that the other gamemodes simply dissapear. OK, Bombing Run was never my favorite, but Assault and (UT99-)Domination always provided much fun