What kind of fish? What kind of filtration (ie, skimmer, # of rock, powerheads, etc)? What kind of maintenance?
If you want medium fish, no bigger than 4-5 inches, you could easily put in 3-4. Or, one bigger fish. Or 6-7 smaller fish. Or 4-5 smaller fish, and one that's 5-6 inches. Several options, but we can't really help without knowing the answers to the above questions.