Can a planet have more than one colony? or is just one and then expand its target resource, to exploit another one.

If you're talking about Stars of Inequity, then a planet can have many colonies. Planets area divided into continents and islands. Continents are further divided into territories. It's my understanding that territories and islands can have colonies.

But you'll find that it doesn't pay as well to have multiple colonies. Stars of Inequity didn't follow the Law of Diminishing Returns on colonies, quite the opposite in fact. Instead of a colony reaching a growth point where the return is worth less than the last investment increment, there's actually an increase.

If you want it to make more sense, you'll have to house rule it.