Finding the "correct" quantity of objects on a map, I think, depends on understanding two things: The function of the map and the basic principles of good design.
The function of the map is usually fairly straightforward--what information needs to be on the map in order for it to be useful for whoever is looking at it? I usually find it is very easy to keep adding details until it becomes difficult to draw from it the information it was intended to communicate. To prevent that kind of overload, I make a list of features the map needs to have and then resist the impulse to add anything that would overshadow those features.
The principles of design are a bit more involved, but I think I can recall enough of my intro to design class to list them: Balance, rhythm, proportion, unity, and emphasis. Google "principles of design" and read a couple of articles. It will probably all seem very theoretical, but trust me--applying those concepts usually results in a more pleasing piece of design or artwork.
That's my opinion. Your Mileage May Vary(tm).