1 or 2 but not for very long. I had my 12 in a 4 foot by 2 foot box and they outgrew it in about 2 or 3 weeks. I wasted time and money building a wood brooder and now im using a cardboard box thats 5 foot by 5 foot. I've learned my lesson and will be using boxes from now on. You can start out with a small cardboard box and just keep adding to them by cutting holes and taping them together.
I built an outside brooder(keep in mind I live in ga so it doesnt get as cold as some places)and it works great. Good space and I built half brooder and half wire with a door so that when it warms up again and they are ready to go out, they can use the brooder until they out grow it. And it stays pretty warm!