I keep 2 females per tank, tried 4 once in a four foot tank and they all wanted the same corner, two killed their litters, other two did fine. Went to 2 each and no problems. I rotate males. I'm not trying to produce huge numbers though. Best production seems to be keeping females apart, less stress, more litters on the ground more you will lose, babies born a few days after ones on the ground can't compete with too many around and starve.