I have some hatchery Buff Orpingtons hens who are probably 4 or 5 lbs. They are not the big, fluffy girls that BOs bred to meet the standards of perfection are. (Although they are pretty fluffy compared to hatchery chickens of other breeds.) I AM looking to get some nice, big hatching eggs from Orpington breeders to put under my girls as they go broody. My question is, how many can I slip under them? I've had 7 or 8 hatch with no problem in the past, but never tried more than that. Is a dozen too many? Fifteen? Thanks!