How much food can be grown sustainably using permaculture methods? In this article on permaculture.com, David Blume recounts his experience:
As far as I know I was one of the only farmers fully utilizing permaculture to produce surplus food for sale in the US as a full time occupation. On approximately two acres— half of which was on a terraced 35 degree slope—I produced enough food to feed more than 300 people (with a peak of 450 people at one point), 49 weeks a year in my fully organic CSA on the edge of Silicon Valley . If I could do it there you can do it anywhere.
I did this for almost nine years until I lost the lease to my rented land. My yields were often 8 times what the USDA claims are possible per square foot. My soil fertility increased dramatically each year so I was not achieving my yields by mining my soil. On the contrary I built my soil from cement-hard adobe clay to its impressive state from scratch....
At most times I had no more than half of my land under production with the rest in various stages of cover cropping. And I was only producing at a fraction of what would have been possible if I had owned the land and could have justified the investment into an overstory of integrated tree, berry, flower and nut crops along with the various vegetable and fruit crops. The farm produced so much income that I was routinely in the top 15% of organic farms in California (which has over 2000 organic farms) in most years on a fraction of the land that my colleagues were using.