Well, the umbrella for this is section 7, "Permitted Uses". Paraphrasing, "any use not listed here is prohibited". The uses of "family", "dwelling unit", and descriptions of N-unit buildings as being allowed in various zones seem to spell it out. But if you really doubt the existence of this rule, I think you'd have to consult a lawyer with land-use specialties.
(Also, 410 CMR 400 has occupancy limits by square footage of units and bedrooms, but they're more generous, usually. 150 sf for the first person, 100 more for each additional, plus bedrooms have to be 70sf for one person or 50sf per person if more than one. So an apartment for five people has to have a minimum of 550sf; not terribly difficult.)
no subject
(Also, 410 CMR 400 has occupancy limits by square footage of units and bedrooms, but they're more generous, usually. 150 sf for the first person, 100 more for each additional, plus bedrooms have to be 70sf for one person or 50sf per person if more than one. So an apartment for five people has to have a minimum of 550sf; not terribly difficult.)