Not many, it does depend on the size of them. Of course you can't put a big Tang in there.
The rule of thumb I will tell you is 1 inch of fish per gallon.
Again, please research what you want, as many fish just won't survive/flourish in a small tank, however there are alot a great looking fish that will.
I like using
http://www.liveaquaria.com/product/a...plies.cfm?c=15, it has alot of pics and gives you the information you need to make a decision.
Have fun!!!