Dono:
Cecil has put a link up before about a "Small Scale Aquaculture" book. If you can find it and get it, it will help immensely. Short of it is, it's not the number of fish in the tank, it's the weight of the fish per gallon of water in the tank. When you size your filtration system I believe it's sized on the pounds (or Kg) of food that you'll be feeding instead of the amount of fish that you have. IIRC it's always sized for the most amount of food you think you'd be feeding, plus some for a buffer.