I currently have a maroon clown, and mandarin goby in my 30G. I've had as many as 7 fish in this tank... lol. The water quality degraded enough that one of my corals was supremely offended, so I rearranged things, the coral approves. I will be only adding one more fish to this tank - just waiting for a super cool fish that will grow less than 3".
What I would suggest is to figure out if you want to have a reef aquarium or mainly fish only aquarium. If you want have a reef aquarium you'll have to keep the bioload on the lower side, or your corals will protest and probably die. And then of course, with a fish only aquarium you can get more.
As Rick mentioned, your equipment will also play a part in your capacity. If you have a wicked awesome skimmer, biopellets and the 9 yards you'll be able to squeeze more in. I don't have the 9 yards, I go with more run of mill kind of equipment and it does the job. If I was still saving up for the best I would be putting fish in my tank in about 2 years...
Your capacity will also depend on your natural biofiltration - live rock, live sand, biostars or whatever in your mechanical filtration system. Canister filters have a bit of a bad rap for being known as nitrate producers or whatever. A mechanical filtration system will trap poop, food, dead things etc. If the filter is not cleaned, then of course nitrates will be produced as the things decompose, so if you can, clean your filter 1 / week. I am totally off topic, I apologize.
Good luck in your fish purchasing!!