depends on size of the fish, activity level, aggression/territoriality, how much waste they produce, how well your tank is set up to handle waste.
in general, you could keep a lot of little fish, or a few good sized, say 5-7", fish. with marine tanks, you can't stock as densely as a freshwater tank.
if you'll post a stock list, folks could comment. you need to select fish that will be compatible, and won't outgrow your tank.
what do you have in the tank now? what do you want to add?