how many mines?

At the moment I am lvl 18. I can build only 1 marble mine and 2 iron mines. Why is that? Is my lvl too low or do I need to conquer more territories?

I own 3 territories and I can build:
5 stone mines
3 copper mines
2 iron mines
1 marble mine

all resources are at a set location. so to get acces to more resources you will need to claim more sectors

thanks!