Depends on the size, temperment, and other factors of the fish. What fish would you like to have in it? A community set-up, or maybe just a couple large fish? If, say, you go with tetras, you could have a few large schools. Or you could have a gourami...cories...loaches...or even a female betta tank!!! *wink wink, nudge nudge*
There's quite an interesting "Community Creator" but use it with caution as I've found that it over estimates the amount of fish (or CM / inches per gallon). Also bear in mind that that's based on a well established tank with immaculate water conditions and good filtration and maintenece.
Edit: I think you might have to sign up as a user to use that creator, however it takes less than 60 seconds for that part.