Just how many you can have depends on the size of the fish, the size of your tank (and associated filter), whether you have seawater or freshwater and whether or not the fish are peaceful together.

As a guide, put just a few small fish in a tank 60cm by 30cm and perhaps ten medium fish in a tank 120cm by 50cm. Your aquarium shop will be able to tell you how many of your preferred type of fish you can keep in your tank.