Is it legal to define rental fees based on a number of occupants?

[Linda Roberts]

Is it legal in Massachusetts to define rental fees based on a number of occupants?

It definitely make sense for me, because I pay a water bill, but I don't usually see it for the long term rentals. Today I found one listing that specified rent per a number of people. I wonder if I can do the same or I'll put myself in a trouble. This listing was by owner, so he may not know all the rules.   

$3200   for 3-5 people.

$3250  for 6-7 people

 $3350  for 8 people.

[Sergey A. Petrov]

What @Theresa Harris said. The landlord may pay the water/sewer bills to the utility company but the tenants should reimburse / pay for it based on either consumption, flat fee, or some other basis (e.g. monthly rent or additional $50/mo utility fee includes up to X gallons of water per month. Overages are the responsibility of the tenants billed at Y rate). The cleanest way of accomplishing that is by installing submeters which is not always possible. Thus a reasonable utility allocation formula.