Large families and groups often have to address the issue of occupancy standards. This issue is especially important when it comes to large families. Federal law does prevent discrimination against certain protected classes.
Familial status is one such class.
HUD's standard is two people per bedroom. If a landlord restricts a two bedroom apartment to fewer than four people he could be accused of discrimination. A landlord can deviate from the federal standard but only for a good cause such as a very small bedroom which can only accommodate one person. A den and sometimes a studio is still a grey area. Building codes also take a part in occupancy.