The planning requirement for the number of people allowed to live in a dwelling unit is based on the definition of a family or household:
FAMILY OR HOUSEHOLD: One or more persons related by blood, marriage, adoption or approved foster care, and up to two (2) unrelated persons; or a group of not more than four (4) unrelated persons occupying a dwelling unit or a group of persons as defined by state law as elderly or disabled. (17.08.010)
Occupancy is also regulated by the building code, so you will need to confirm those requirements with the Building Department.