For example, 6-12 Neon Tetras and 6 Cory Catfish would make a great beginner tank.
One of the keys to a successful aquarium is not over stocking it. Too many fish in a small or large tank will make it more difficult to maintain and prevent other issues such as nuisance algae. You may have heard to the X inches of fish per gallon rule but I never liked that concept. For a 20 gallon aquarium, you're going to want to stick with small fish close to the size of a neon tetra. For example, 6-12 Neon Tetras and 6 Cory Catfish would make a great beginner tank. Less is more, trust me.