Depends on the climate; also depends on your goals (ranging from "keeping them alive for a couple years" to "avoiding really-high risks of cannibalism" to "happy relaxed chickens")
A good all purpose answer would, quite seriously no foolin', be: "as few as you can manage to content yourself with".
Good luck, have fun,
Pat, having done 4 sq ft per hen indoors (plus ample run) and up to 15+ sq ft per hen indoors (plus ample run), and having SEEN how differently they behave with the larger amount of space, would never do less than 10 and I generally do 15 sq ft plus run. And no, that isn't because of my climate, I'm comparing their behavior in the *summer*.