It depends on the world you're in. The select world screen tells you how many you're allowed. It'll be something like this:
4[1]
What that means is that you can 4 domestic hubs and 1 foreign one. So something like this:
6[2]
Means you can have 6 domestic and 1 foreign hub.
Also remember that 1 foreign hub = 2 domestic hubs. So if the number on the left is 6, you can only have one of following combinations:
2 domestic and 2 foreign
4 domestic and 1 foreign
6 domestic
Capish?
