In a 6m x 6m area, the chickens stop laying/hatching at 10.

I have not tested, so maybe for you to try;
3m x 3m limit 5 hens?
9m x 9m limit 15 hens?

This is VERY inaccurate and global estimation. I just know I have 10 hens in a 6 x 6, and they are happy, and stopped laying eggs. Stupid cluckers.