I found a website that lists Mass laws related to tenants and landlords. I don't see a reference to restriction of number of occupants by the number of bedrooms but it does say the following:
Every dwelling unit shall contain at least 150 square feet of floor space for its first occupant and at least 100 square feet of floor space for each additional occupant. Every room occupied for sleeping purposes by one occupant shall contain at least 70 square feet of floor space; every room occupied for sleeping purposes by more than one occupant shall contain at least 50 square feet of floor space for each occupant.
http://www.mass.gov/Eeohhs2/docs/dph...10.pdf#page=14
The following may also be useful resources:
Massachusetts Tenant's Handbook
Massachusetts Tenant's Handbook
Massachusetts Tenant Rights
The Massachusetts Tenants Network
I hope that is helpful.