Legal Question in Landlord & Tenant Law in Massachusetts

A Landlord can restrict guests staying over in someone's unit but the restriction needs to be reasonable. I am not sure that 2 days is reasonable. Moreover, the restriction should serve a business purpose. Clearly staying over for months is unreasonable without the consent of the landlord. However, if a tenant has a sibling that wants to visit for a week is that unreasonable?

If someone is dating someone seriously or is engaged, do you think 2 days per month is reasonable?

You have another problem depending upon how your lease is written in that your rules and regulations cannot be so restrictive as to change the use as of the date the lease was signed unless your lease provides for it.

Without knowing the size of units or the character of the premises being rented it is difficult to give you any substantive guidance.

You certainly can change the rules for NEW Tenants or for lease renewals to no more than 1 over night guest per month and no more than 2 nights-- but that seems a bit restrictive and you may find it difficult to find tenants. Absent a compelling reason for such a restriction, it seems a bit unreasonable.