Hey Shezz,
I cant really give you any hard, fast numbers for the fish population you can keep in your tank, for the reason's Nikki and I mentioned above, (size of fish, predatory or not etc...), what I can do is give you a loose kind of guideline....
The guideline is this.....one inch of fish for every 5 gallons of water in the tank. So in a 55 gallon tank, your looking at being able to keep 11 inches of fish. Another guideline for you is to look at the max size a fish is going to get, and make sure you have at least 6 times that max size for the fish to be in for the tank. For example, if you are looking at fish that gets to be 12 inches in lenghth, it shouldnt be housed in a tank any smaller than 6 feet in length.
Make sense?
Nick