Question about breeding rats

Im using 20 gallon tubs on my rat rack. How many females with litters can I have in a tub? I had 4 females with two litters in a tub and right as I was getting ready to move one litter that had just been weaned I noticed they were killed. I was just wondering why this may have happened? Should I only have one female with her litter? I also noticed that there were two pinkies. One was dead a very shriveled. The other alive but shriveled and you could tell mom wasnt feeding it.

Any ideas or suggestions appreciated. Im pretty new to breeding rats. Thanks.

Oh and they have plenty of food and fresh water. So its not hunger.

Are they first time breeders?
Make sure they have plenty of food & water.
I would seperate the mothers into individual tubs, to find out who is killing their babies.

what are you feeding?

they are first time breeders, and I use Mazzouri block for food.

almost all rodents are known to have cannibals in the species, so its probably a single rat doing it, and like llovelace said above, seperate the moms into their own tubs