A good general guideline is to stock 1 inch of fish per gallon of water, so a 10-gallon tank could hold a combination like ten 1-inch fish or five 2-inch fish. It is the adult size of the fish that matters; many fish in the store are not fully grown.
The First Tank Guide recommends using the inch-per-gallon rule only as a guideline. Additionally, Tropical Fish Hobbyist Magazine advises stocking a 10-gallon tank only with small, inactive and non-aggressive fish. Carefully research the size, habits and behavioral needs of fish species before stocking a tank to prevent predation and injury.