how many in a group?

I find that with more than 3 girls, you're likely to have more crushing or suffocating of the babies by the older pups that are still not old enough to come out. That could be because the females aren't completely synced up with regards to their birthings. I have had a colony of 1.5 for a few months now, and the numbers are ok, average like 7-9, but nothing spectacular. I get far more from my groups of 1.2 or 1.3.

Just my .02$

1.3 is my norm with those breeding groups living in nice roomy 30 gallon tanks with a wheel and a house and so forth. Any bigger breeding ratios than that it seems to be less than completely productive for me.