how many babies do moms usually kill.

[Shadera]

Mice are skiddish creatures to begin with, she may have culled them due to some deformity, or they may have died, then she ate them, who knows. If she is doing well with the others, keep her. Is she a first time mom?

I agree with this. Sometimes up to the entire first litter doesn't make it for whatever reason. I give them one more chance. Usually a baby eater eating them just for the sake of eating them will eat/kill the entire litter and not just a couple. If she kills them all a second time, into a snake tub she goes.