A minimum of two, for welfare reasons. However, if you're talking about ex bats, up that a bit just in case one or two don't make it, so you're not left with one on its own.
The upper limit is dictated by the space you have for a run, or how much you like your garden to actually have grass and plants etc. Hens are addictive though, so Bionic is right - start with 3 or 4 and go from there.