If a household has a single individual, he/she must be marked primary, but if there are multiple individuals associated with a household, only one of the individuals associated with it can be marked primary. An individual can be associated with multiple

@Sundi Chaithanya Reddy - Are you referring to NPSP? If so, a contact (individual) can only be associated with one household (not multiple households). However, they can have an affiliation with multiple organizations (accounts that are not households). As far as households, you're correct in stating only one household member can be marked as the primary contact for that household.