Each foreign base (as in, a different continent) takes up two of your base quota (domestic bases only take up one).
So if you allow 10 bases, 5 foreign, people can have: 2 domestic, 4 foreign; 4 domestic, 3 foreign; 6 domestic, 2 foreign; 8 domestic 1 foreign; or 10 domestic bases.
You must have one domestic base, so with only 9 bases left in your quota, you can have a maximum of 4 foreign bases.