The tractor you see in the photo is approximately 8x8ft inside and I typically keep 10-12 dual-purpose size birds in each one. I try to move them every day so I don't let them out unless I'm doing something like planting winter forage as I did there. I favor at least five square feet per bird in the tractors. If you'll be letting them out to free range every day then I'd put as many as twenty in each one since they'd mostly only be inside to roost, lay eggs, or get to the feeders and waterers.