I think after 4 litters they should be retired. Five if they are going strong. Back to back constant litters may be convenient, but she will produce healthier, fatter babies and more of them if she gets time to rebuild between litters. And back to back breeding is hardly humane.
They don't even live long enough to produce more than that in the wild, and birthing is very taxing, health wise.
Hold back the babies of the 12+ litters, and your stock will usually have more babies if you treat them right.
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