What is the PA state law regarding the number of tenants that can live in a two bedroom apartment?

It is up to the landlord and possibly the building code inspector
There is no real law -that I know of.
Generally the rule that HUD/sect 8 and Children Youth and Family go by is two per bedroom
Parent(s) one bedroom and two boys per bedroom and two girls per room
4 for 2 bedroom apartment

I can't find a statute governing that for the state. This is usually a matter of local housing codes.

This is usually something determined by municipal ordinance. You can contact the zoning department in your town to find out the details.