The nice thing about having players in all primary groups in the RP is that the numbers can be scaled with the player/character count. When one faction grows beyond a certain size in comparison to the other two, they can be scaled up. If there is an excess of prisoners, new Sith and rescuers can, and will be permitted.

Or, of course, people end up dying. You never know for sure. But the deaths could, of course, just as well be in Sith and rescuer ranks as in prisoner ranks. That, naturally, will open up slots as well.