The amount of fish you can fit into a 55 gallon aquarium comfortably is not based on the amount of water, but the size of the fish. The general rule is 1 inch of fish per gallon. You must take into consideration the size the fish will be when it is full grown, not the size the fish is when you get it.
For a 55-gallon aquarium, you can add 1 fish per gallon. This, of course, depends on the size of each fish. To ensure comfort for large fish, do not exceed 17. For smaller species, you can add anywhere from 20-50 fish.