What, in the end, do I really need?
The answer to that question usually depends on the scope of your project.. So I find it best to just start out with the minimum number for where you project is - in my project's case, a regional hub map, several buildings for the academy the characters are attending, the several neighboring towns around the academy, and those towns' most important landmarks.
Whether that's enjoyable or not depends on a few factors - what RM version you're using, how many events there are to track, what kind of puzzles you intend to be there, what bugs you're running into, and how many times you need to redo them just to name a few.