How many fortresses to build?

[OgreMkV]

Here's what I do (and please understand that I'm a relative noob).

I'll expand until I find some good defensive choke points (uncrossable mountains, oceans, etc). This should be between 7 and 10 provinces, depending.

Then I'll fort up. Put a fort every other province or so, in the important choke points, in good income provinces, etc.

After that, I build up considerable armies (both recruiting and magically), then expand to the next chosen points. Again, choke points, other players forts, etc.