If they have
enough room to roam in a healthy ecosystem, yes, they will happily feed themselves. Make sure you've got a good breed, the production meat and egg breeds can 'sometimes' be bad foragers. But most chickens would probably adapt and figure it out. Don't get more than your ecosystem can support though. Depending on your zone and winters, you may have to at least supplement in the winter. I've had chickens roam about a 4 acre territory. Turkeys a bit more. I've had birds sustain themselves on fallow pasture
land without any additional feed, even raise their chicks just fine. How many birds your land can support depends on a lot of things. How lush is it, what is growing there, is it flat or does it have elevation, what systems are you implementing to help feed them?