Thank you for your question. The amount of fish you add will depend on the type of aquarium, fresh or saltwater. For salt water a general rule is to stock no more than ½ an inch of fully grown fish per gallon of water in your aquarium. For example, if you have a 30-gallon aquarium, ideally stock no more than 15 total inches of fully grown fish.
However, for freshwater a general rule to follow is to add 1" of fully grown fish per gallon of water. For example you would be able to add 30" of fully grown fish to your aquarium.