A 2nd fort in the early game is critical I've found. Any more after that depends on your income, nation and situation. A rule of thumb is that if you can't afford to recruit more than X mages per turn then its not worth building an (X+1) fort. This will vary depending on cost of mages, some nations get good cheap mages that are useful in battle so a 3rd fort early will be a big priority.
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doesnt it also give you more income
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Yes, but generally it isnt worth building a fort solely because of income, because the cost of building the fort versus the number of turns to recuperate the cost of the fort will be at least 15 turns, usually a lot more. Most of the cheaper forts have a low admin value so you dont get much.
Your primary considerations should always be for recruitment purposes, either for your own national mages or provincial mages.