there wasn't a set amount of members, instead, they were interchangeable with every comeback or debut of a new unit. they weren't bound to by any contract either, so if they really wanted to, they could leave at any given time. the popularity and how well received one was, would ultimately amount to their exposure and time in the limelight; because what the company really wanted, was to milk out as much money as they could from each and every one of them.