how many fish?

The general rule is 1" of fish per gallon of water. So, 55" of fish.

As far as I understand the inch/gallon rule is a bunch of bull****. How many fish you can put in depends on many things, the most important of which is the ADULT size of the fish.
If you want small and colorful why don't you try doing a couple schools of different tetras with a 3 or 4 corydoras, otos and/or snails as a clean up crew.

The best thing is to pick a centerpiece fish (or something you really really like and is a must have), read up on it to make sure it will live happily in your 55g. Then find out what makes good tankmates with it. If you want a cichlid then you would have to be careful about what you choose. If you want peaceful community, then a dwarf gourami and some tetras will be nice. You might also be interested in a livebearer so there is always some fry in the tank (fun to watch) so some guppies, platies, or swordtails might be nice. Its all up for what you like . Just look into have a mix of bottom feeders, mid-dwellers, and top-dwellers.

Just remeber when buying fish, the size the will grow to!