Broodiness is a hormone change and they can go broody with ho eggs at all under them -- and sit on no eggs for weeks and weeks. It takes 21 days aveage for a chick to hatch, by the way, at least in chickens.
If it were me I'd remove the eggs I wanted to hatch and save them in the house for up to a week, maybe a little longer, (put them in a carton and tip it twice a day to turn the eggs,) then put them all under the hens at once, so you won't have a staggered hatch, where the nens will leave any unhatched eggs to raise the chicks.) I'd also watch the hens to be sure they stay on the nest 24/7 except for one brief outing, at least one overnight without roosting, before I called a hen truly broody
They will do best if they each have their own place to set, their own little pen with nest, food, water and a little room to exercise and dust bathe. this way they won't steall eggs from each other, lay in each others' nests, squabble ofer nests, etc. But you can try it with them all in the same place, you're just likely to get a few fewer chicks this way. It is the way hens have been doing it for hundreds or thousands of years, after all.
I let mamas raise chicks all in the flock, once they hatch and are up and about. Just take any layer feed out and put some oster shell out for those who are laying. They will all be fine on chick starter, grower, flock raiser, whatever, even medicated chick starter as long as it's medicated with amprolium (and you can eat the eggs.)
And is this healthy. here are 3 of them
