Oh, don't forget about location. Where the city is can also effect the number of guards. This could also be done with army troops but they are normally for patrol outside the city but always fun for conflicts.
You can also go with wards/districts with each one building their own police force that answers to the city guard. Another area of conflict but you get games like the Three Musketeers. This is great for larger cities.

You can also go with wards/districts with each one building their own police force that answers to the city guard. Another area of conflict but you get games like the Three Musketeers. This is great for larger cities.