3 is probably max it would shelter healthily...especially in your winters.The inside is fairly large. I got it on sale in a pinch-I took in a rooster who was dumped on my property. Then got a few hens because why not? The 3 that were in there slept comfortably with room to spare. I don't have any problem closing it in though. My only concern is having enough room to be happy. They love the free range obviously but I live on 6 acres with 2 farms on either side and a 500 acre park across the street so lots of predators
Check this out: Fantastic treatise to help decide how much SPACE your chickens need.