Andrew,
I have had this same question come across my desk when dealing with a property purchase. In Texas, I contacted one of the local Apartment Locating Companies to obtain a unit breakdown. Most Locating Companies have a unit matrix that breaks down each floor plan style and the total number of units on property. Each state should have its occupancy limit per apartment, usually this is something the Fire Department will have if you are not 100% sure. In Dallas, the rule is a maximum of 2 persons per bedroom (Studios and 1 bed = 2 persons, 2 bed = 4 persons, 3 bed = 6 persons), and a 1 bed/study allows for 3 persons seeing as the study can be an option small bedroom as long as there is a door that can close.
Once I have the unit breakdown, I can estimate a high and low population on the property. This is the best way I have found to get an estimate on the total population and then an average per unit. I hope this helps!
