A key thing as well as the size is how easy it is to keep clean as hens produce a lot of poo and it gets slippery very quickly if they are in the run all the time, mine free range..my little coop was hard to clean so I bought an 8x6 shed with plastic stuff on the floor, they mess that up very quickly, I have 13 hens and my first lot used to roost in the trees until I moved a bench. If you only have a small run and hut, 3 sound an ideal number!!! I also used fallen brances from my conifer trees in thier run but no conifers now!!