just one. If you want two clowns get the same kind in a tank that big. I would not personally get more than 2 fish in a10 g tank. You can if you want, but you will h ave to do VERY FREQUENT water changes, and keep a CONSTANT EYE on the water params. One fish, a shrimp, and inverts would be my suggestion, and that is if you have lots of rockwork for a 10 g.