I voted - max: 20
10v10s are more fun on mediun and larger maps. also: lots of people + quick respawn = chaotic fun!
-another idea- have different servers run different limits and have different map rotations that are best for those limits. For example:
[pub1]-max players:8, maps: oddjob, theice, shock, ubcliff, back2back...
[pub2]-max players:16, maps:airtime's, shazam's, nitefactor...
[pub3]-max players:24, maps:*any map that's big, like pb_shreds.
[pub4(chaotic)]-max players:32, maps:*some from [pub1] with guns that are Spyder SE or better, spawn time:3 sec, 50 nades/team