How many fortresses to build?

[Sencha]

I feel like I'm getting better at the game. But with each success, I think I could do better. This time around I was playing Ermor, in the MA on a medium map, with four other opponents. It was year three and I controlled 18 provinces and seven fortresses, mostly scattered around my border. I had more than 18 lands before, but the AI was getting more aggressive.

I noticed that with the increase of fortresses, I could raise bigger armies in a shorter amount of time. I also noticed much less of a resource shortage. However, the AI was still doing better comparatively, and my intelligence on their regions was low. I didn't know how many fortresses they had, but I was thinking I didn't have enough.

How many fortresses should a player try to build? Is there an ideal number, or does it depend on the conditions? I try to consider the layout of the land, and the terrain type that I'm building my fortress on, as I know that can effect the type built and resources gathered from it. But perhaps I should have had more fortresses than seven?

How do other Dom3 players plan their fortress construction/number?