Mom taking on a surrogate litter

Question
Hi there! I'm a volunteer for a local cat adoption agency and have been fostering for some time now. I currently have a mom with a litter of 4 babies (approx 3wks old) who was found outside in the snow where she had given birth to these babies. All are doing very very well. They are gaining weight, look healthy and although mom is thin, she is eating and drinking a lot. My question is a littler of 6 neonatal just came in as well (approx 4 days old now) without a mother. I know mom could be a surrogate to this litter, but my thought is that adding 6 is way too many. How do I know how many babies I can add to her litter for her to feed? I don't want to jeopardize mom's health or the well being of her current litter. Can she take all 6? Can she take half? hopefully finding another surrogate for the other half? Can she take any on? Will mom begin producing the milk needed to support 10? Currently the 4 day olds are being bottle fed by another foster mom who is lacking severely on sleep now and not only would she benefit, but the babies would benefit from real moms milk as well.
Any help/advice appreciated!
Thanks,
Lisa  

Answer
Hi Lisa.  Ten would really be too much for her to nurse exclusively.  General rule is, 6 kittens is the absolute max for one mom.  Any litter greater than 6, even if they are all biological, needs to be supplemented with formula.  I think it would be okay to give her another kitten or two, but I wouldn't do more than that unless you can supplement the kittens' feeding with formula.  

In my opinion, the ideal situation would be to keep the fosters separated from the mother all day, and bottle feed them with formula every 2-3 hours at this age.  Then the kittens could go with the surrogate mom overnight for care and feeding.  At least this way, whoever is bottle feeding the kittens can get a good night's sleep, the demand on the foster mother's body won't push her over the edge, and the kittens would also get the antibodies and contact with mama that is so important to their physical and emotional development.

Hope all works out!  And thanks for volunteering to make a difference in the lives of these kitties.