The problem is either you cannot get the number perfect and so you must make it too large thus doing nothing over not setting a limit at all. If the number is too low you will discover next election people who think they need to park (they may or may not be right) are mad enough to vote you out and undo things. By just not setting a limit you let every property owner decide for themselves what is right - and if they discover they are too low they can hire someone to build more parking (at their expense), while if they decide they have too many they can replace parking with something else - like another building.