I have 4 nests and have had up to 18 layers...worked fine, not that there weren't some squabbles at times.I always read it was good to have one box for every 3-4 hens. I guess I'll have to keep an eye on them once they actually start laying to see if anyone picks someplace on the ground instead of the nice, curtained boxes.
New layers may lay anywhere at first.
Curtains are for the keepers more than the chickens.