I think my largest was 19 that all survived. I do like to foster some off in a huge litter if I have a female that has some the same age and not so many in her litter. If not, then sometimes you get a lot of baby rats but they aren't growing as fast or as well because there is such a fight to get to the milkbar. Your female will split them up into multiple nests to help with that but if you can help her by fostering some off, it usually helps in getting bigger pups.