How many fish you can keep all depends on the type of tank and how often you want to clean it.  If you have a reef tank you are not going to want more then 1" of fish for every 5 gallons.  If you have a fish only tank you can get away with 2"-5" for every 5 gallons.  Ultimately this decision will come down to fish species, maintenance, and amount of corals in the tank.