A landlord can only limit based on the local munipality's guidline of maximum occupancy per square footage. A Landlord can not limit based on any other criteria as that is discriminatory. Check with your local municipality to see what limitations they have and that is all you can enforce. Due to fair housing you do not have a right to chosse based on the familial status of the occupants(e.g. children) only the number based on local laws. The best way to word this would be "no more than x number of occupants as per local ordinance"
Hope that helps.