I use 8 of them and so far so good. Our biggest issues are great horned owls, racoons and coyotes. No losses in the last year, except lambs to coyotes well beyond where I had installed the night guard.
I started using them on the suggestion of another farmer, who has nearly 1,000 laying hens pastured. His biggest problem was owls, so he places his atop a 15' high boom coming off the hoop houses.
He and I both place one on each side of a post, so they point in each and every direction. So, you really need 4 per installation.