In the middle of August, I put 15 standard sized chicks (hatched in an incubator) under a standard sized year old Black Australorp. All 15 are alive today. SHe raised them with the flock and they are now on their own. She had a sheltered place to keep them overnight (the floor of the coop), they free ranged with the flock during the day (she found a place kind of protected from the wind where they hung most of the time), and the weather was pretty warm. She could cover all the chicks at one time if she wanted to for the first week and a half or so, but after that, she could not cover them all. Even before they got too big for her to cover, some would be out from under her at night but snuggled up close. I'm not sure what they did if they did get cold.
It is later in the year, I don't know your weather or what conditions Mama has to work with, but mine had no real problem with 15 standard chicks under a grown standard hen starting in mid-August.
