12 babies certainly isn't too much for one rat mommy, if that's what you're worried about. As long as the babies are getting fed (you should see white bands on their tummies), and are being watched after by one of the mommies, I wouldn't worry too much about it. Seems like mom #1 just isn't interested in being mom. If there is no fighting, and babies are thriving, don't worry, it'll all turn out ok.
