There's a bunch of answers to that... You could keep 30 in there, but I dunno how happy and healthy everything will be. With an average of 2-3", I'd say 15 should be good, if you're question is pertaining to bio load on the system, assuming you've got a decent amount of live rock. But depending on what species and how territorial, the answer can change. It all depends on the type of other fish, amount of live rock and layout of the tank. I think pushing the limits of the tank isn't a good idea. If everyone is happy and has their space, everything with be much happier and less stressed.