You can avoid a Fair Housing Complaint by allowing 2 persons per bedroom. Some also allow one extra person for their rentals. Be sure not to restrict the number of adults. For example, do not limit occupancy to two adults and two children in a two bedroom. You also need to allow four adults or three adults and one child.
The bottom line is you need to require at least one adult, but after that it could be any combination of adults and minors.