There are a number of ways to do this, but:
1. The whole point of the system is that models are judged against an objective criteria instead of each other. This means that you can give out any number of any colour of medals per category (and yes, zero is a number). If you have four amazing gold-tier models in a category, you can give out four gold medals.
2. Since models aren't competing against each other and you can have multiple medals per category, you don't need as many categories in order to have like competing against like and a reasonable number of models per category. You could simply have a few categories, such as aircraft, armour, automotive, figures, etc.
3. One thing to consider is whether you want to judge every single model or a modeller's work within a category as a whole. By this, I mean if I enter eight models in a category, should I get one gold, five silvers, and two bronzes, or should I just get a single gold to represent my best work? There are pros and cons to each; the second way of doing both cuts down on award expenses and doesn't drag on the award ceremony, but it means that entrants have to put all their models within a category next to each other in a little group so you can tell at a glance which models all belong to the same person.