Even through attrition from feeding them off the mommy rat will take care of any/all babies in there regardless of numbers - they will even take another rat's babies and raise them as their own (not sure if the moms having been previous cagemates has anything to do with that) but they do not seem to differentiate between their own and others - seems like anything little and squealing for milk gets some luvin.

This of course in my admittedly limited experience.