typically a rat will not kill offspring but rather clean up the ones that pass or have something terminally wrong with them.... that could be what is happening with this litter. If the numbers continue to decline then some questions written below would need to be answered in order to gain a better insight as to what is going on
there are rare cases of a litter being killed off by a mother & that is typically the result of the following: it may be the result of a stressed mother, a fearful mother, a mother that is too young, a malnurished mother, it could be an issue of her not lactating or things along that line. The mysterious cases are often along the lines of genetic issues & at that point it is all guess work without first hand knowledge of multiple generations of the lines involved.
I have had females care for large litters of 18+ with no loss. I have had large litters that reduced in size by a few the first few days to what I assume was sustainable levels after the weaker pups passed.
Momma-rat only has 12 nipples so it is survival of the fittest. Often a female will split a large litter into 2 feeding groups or nature takes its course & everyone seems to get what they need.
The only way she can keep up with a large litter without loss is to be in peak health & to have a proper diet so that she can produce milk to nurse the young.
I could ask a whole list of questions as to her age, health, environment, diet provided, first litter, has she had back to back breedings, history of her line, history of the male's line, etc, etc, etc
Hopefully something written here helps answer some questions you have, if not go ahead & answer some of these general questions written above.
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