8-12 inches?? maybe 8 with a very regular water change schedule and exceptional biofiltration. In that case, aggression will begin to become an issue. In reality, as long as you don't have aggression issues, you could put 20 of them in there, provided you were changing 50% twice a day w/ a couple Fluval FX5's on there.
Long story short, there's not enough info here. How much maintenance are you willing to do, and which fish do you want to keep? Blue acaras, or discus, or angels(4-6 in.)... a couple dozen. Oscars, managuense, flowerhorns (12-16 in.)... maybe 4 or 5(with a standard weekly 50% WC schedule).