I have to say it depends how old they are. I had about 20 slow growing birds in an 8x8 tractor who were 5 months that didn't tear up the grass. The difference between them and the meaties was that the meaties poop so much more than the conglomerate of poo in the morning can sort of pack down on top of the grass, which is not good. When they're little (4 weeks) they don't scratch or poop as much, and you can put 40 in an 8x8 tractor. As they get larger, they put more stress on their environment. I know that we know this intuitively, but it is useful to think about when planning.