There ARE Limits
Everyone, both landlords and tenants, need to know that there are County imposed limits on the number of people who can live in a home. Generally, square footage of bedrooms is first, but so are the relationships between the residents.
Related people, husbands, wives, children, grandparents, etc. fall into one category. The other category involves unrelated adults. For the details of two County Zoning Ordinances:
Fairfax County: CLICK FAIRFAX
Loudoun County: CLICK LOUDOUN
Generally the limit is no more than 4 unrelated adults in a home, but take a look at bedroom size, access to bathrooms, and emergency exits or contact your local zoning board.