I belive most people underestimate the number of fish an aquarium can comfortably hold. The important factor is to be certain your fish will get along well together. As long as compatability is not an issue, and you are careful to select fish which you know you can care for, I see no reason you cant have 6 fish the size of a Coral Beauty Angel or smaller, or 4 fish the size of a Porcupine Puffer.
A good selection for a beginner 55 gallon might look like this: 1 Coral Beauty Angel, 2 Ocellaris Clowns, 1 Six Line Wrasse, 1 Sharpnose Puffer, & 1 Kole Tang.
Keep in mind, more fish = more frequent maintainance and more sizable water changes. Avoid fish of the same color or fish of the same family, with the exception of keeping a pair of clownfish.