I think it depends on so many different things that there is no magic number that covers everyone. Check out the link in my signature to get my thoughts about some of the things you should consider.
That 4 and 10 that people so love to quote is a reasonable starting point if you have a small flock of just a few hens all the same age in a small urban lot, though weather, breed, and the temperament of the individual chickens play a role in that. But if you start mixing sexes and ages, have broodies hatch or raise with the flock, or plan to integrate the space requirements change rapidly.