It doesn't get that cold in my area, but I let broodies hatch year-round and never give a heat lamp. Right now I have a Welsummer with 13 chicks and they all pile up under her at night. I think the key to very cold weather is to give the broody and her babies a space as draft-free as possible.
I don't agree that chicks feather out faster with a hen than in a brooder. Biology is biology. What I do believe is that chicks can handle a lot colder tmperatures than many people think as long as they have a place to go warm up when they need to. Doesn't matter if that is a heat lamp in a brooder or a warm mama hen. I do think chicks that are exposed to cooler temperatures can handle cooler temperatures better than those kept in an oven all the time.
I've had very young chicks play in the corner of the brooder that is at 70*F, then go back to the heat lamp when they need to warm up. I've had very young chicks roam around outside when it is a lot cooler than that and go to mama when they need to warm up. Warm up is not as often as you might think.
I've raised as many as 15 chicks with one broody. Temperatures were well below brooder temperatures, say down in the upper 50's F at night. She could cover them all until they were maybe one to one and a half weeks old. But when I would look at them at night, not all would be under her even at that age. A few would be in top of her or snuggled up next to her, not hidden under her. I think they take turns going under and coming out. And it is not a case that she needs to be able to "cover" them all like eggs. I've found chicks up off the ground under a broody's wings.
If it is really cold, I'd cut back, but I agree that if she can cover the eggs, she can pretty well raise the chicks as long as she is not in extreme conditions. She obviously does need a draft-free place.
i gave my bantie cochin 22 chicks standard cochins bantams cochins and sebrights
she loved them and covered them all till they were about 4-5 weeks then they couldnt all fit so like ridgerunner said they snuggled up next to her or on top of her
When I asked the question I was referring to 12 standard Ameraucanas under a bantam Cochin, and that seems like it will be okay.
I also have 15 Serama chicks due to hatch next week that are currently split between a Serama and a Cochin hen. I wonder if the Serama hen ("D" size) could keep 15 chicks warm for 5 or 6 weeks? Is that pushing it?