Record so far....

In the short time I have been breeding mice (less than a year at this point), my girls average 8-12 babies.

I just had one deliver 15 this last week. o.O

hold back some of her daughters for breeding, might be able to increase your average litter size.

I am ok with where it is at. I only have 1 snake, but I work with someone that is buying at least partially from me. She has baby corns that are taking pinkies right now.

So really, I am already breeding way more than I need. I am good. But yeah, I would totally continue that line if I needed more!

And now I had a mouse deliver 19 babies. The runt was incredibly tiny. It is a good thing that I have someone buying pinkies from me, because I doubt that many would survive to weaning.

My black female (my wife named her Midnight) had 11 babies.
5 days later I had a gray one (wife named her Silver) that just had 16 or 17 babies... was hard to count them.
I had a Tan mom that had her babies the next day. This was the Tab's 2nd litter, she ate all of her 1st litter. Note - I also saw her kill a different female's babies a month before. I still had her in with a male (my nicest male), so I took her babies (5) and gave them to Silver and Midnight. I noticed that she pushed out 4 more, so I left those 4 with her to see how she would act. Next day, there was 3, then next day was down to 2. I know that if a mom has only one or 2, they will not lactate enough.... so I moved her and the 2 babies in with Silver and Midnight.
All 3 mom's were helping feed call the babies, but I noticed some dead. I know this happens so I just left em and decided to watch... Then I noticed one of Midnight ' babies with its head chewed on.... I just knew it was that Tan little butch killing them. So I removed her.... and have had no more dead babies. She had to have killed a bunch because I ended with only 19 babies.

Sorry for the long story, was mentioning that my biggest litter was 16 or 17, although not all are alive due to that Tan. Tan is prego now, and after she has babies, I am stealing her babies to give to a good mom. Then she is meeting my ferret.

I actually switched to a line of mice that's pure white and gives up to 19 babies per litter. I got rid of all my fancy mice because they were not producing that much. I like to run several females in the same tank, the seem to pile up all the babies and then take turns in a spread eagle position over the pile trying to nurse them all LOL.

I usually have 2-3 females in tub together with babies too. They all help feeding the babies.
I had 3 together, 1 wad still nursing the babies, even tho her babies were 5-1/2 weeks old. I just removed her yesterday as some of the varies (19 of them) are starting to get weened and I figured 2 moms can handle them.
I do have 3 females in with them too (8-1/2 weeks old).... they are not nursing the babies, but I believe that they are "learning" to be good moms. Plus I think they are learning that my hand touching the babies is okay (for when they do have their own babies).