
As in humans milk doesn't come in properly until about the third day, which is why in the first three days the pups are almost constantly suckling and the bitch never leaves therm except to be forced to go to the toilet.
After the milk comes through in more volume pups actually get more each feed and there are more obvious feeding and sleeping periods.
I would never remove any pups to be hand reared, would sooner have to top them all up several times a day and they are then still getting the benefit of mothers milk. I would constantly plug in smaller pups whenever I came near the bitch.
I would avoid any additional feeding if pups all look content for the first few days, even if a little weight is lost.
I have a medium size breed (20kg adult weight) and one bitch my friend bred had litters of 14 twice (though reared 13 in one) and she had plenty of milk up to normal weaning age.
If you hand rear some of the pups they won't benefit from Mums milk, and also it will be much more work (every two hours around the clock) for you and hardly any less for her, and might upset her and stress would affect her milk negatively.