I assume you are talking about day old chicks. They need about 1/2 square feet each for the first week or two, but they rapidly outgrow that. Then, usually, they need about 1 square feet each for another three or four weeks, but watch for cannibalism at each phase. You don't want cannibalism to start and you have to deal with it immediately if it does.
This is for full sized chickens. Banties can get by with less. But more is always better. Having extra feeders and waterers can also help sometimes.
I have 13 3 week olds in a large cardboard box that is about 3ft by 4ft and everyone is doing fine. I have a medium feeder and medium waterer in there along with some sticks for them to learn to roost on and that's it. I have been starting to give them scraps of food to play with to keep them busy. I let them out on warm days so they can start getting used to the big world outside the brooder.
Quote:I have 18 3 week olds in the same size brooder. Theyre doing just fine.
wanted to add, pecking is natural for chickens, it doesnt usually start out as canibalism. I give my chicks some cleaned off coins to fight over, and a scrap of heavy paper too. this keeps them quite busy.