The rate of colonists depends on your number of ports and a couple other things, it's not fixed. And the people you send into the colonies don't just come from your own lands. This is not Victoria II, you don't play a nation state. Lots of people may be willing to take a ship to your colonies and live under your benevolent rule.
All you do as the state is you finance the ships and the organization of the colonial effort. You don't pick who should go, or where they should attract colonists from.
But somewhere they have to come from. The colonization-system in EU 3 creates populations in colonies out of nowhere. So if colonists are not from your own country but from a 'world-pool', this pool should be restricted. No more unlimited colonizing should be possible. Or it should at least be expensive to do so. Spending 15gp for a colony + upkeep is way to cheap.