reference "How to Grow More Vegetables" by John Jeavons
Ecology Action has done a lot of work on how much land it takes to feed a person a vegan diet. They have determined the smallest amount is about 4000 square feet per person using Biointensive techniques. This includes land used to grow compost crops, some of which are grains. It includes no land for animals or animal feed.
Ecology Action: http://growbiointensive.org/
Eliot Coleman determined about 2 acres of intensive vegetables is the most one person can manage commercially.
reference "The New Organic Grower" by Eliot Coleman
his farm: http://www.fourseasonfarm.com/index.html
I read an article a few days ago where a man seemed to have done a paper on how much work one man can do. He was able to provide enough for 2.5 people in one year. He used a tiller. I am using a shovel.
That sounds interesting, got a link?
If you are trying to grow with the same methods as the man with a tiller, then yields will probably be less. Even if you produce as much food/energy, you will be burning more to do the work of the tiller. But growing annuals and tilling the soil is not so efficient ... cardboard mulch is a low-effort alternative in many cases. Trees are better still.
As a counter-example, my grapefruit tree yields enough fruit for several people to eat 1 or 2 large grapefruit every day for 5 to 6 months, and the work is close to zero (mostly harvest and eating). That is only 100 to 200 calories (5% to 10% of a day's need at 2000 calories a day for light work). But if that one tree is multiplied by 10 or 20 with other trees or efficient types of plants, there is your diet for several people with relatively little work. If your up north, you might be growing apples and pears instead of grapefruit and tangerine, but again, one or a few trees can provide 1/10th of a person's needs. The big disadvantage is that grains can take a farmer from zero to sixty in one season, while trees need a few years to build momentum.
Sure, a permaculture system needs a similar high yield with crops for protein and oils ... but that is possible. You mentioned nuts, which are a good way to provide some protein and 20-30% of calories from another single plant. Most legumes are annuals, but there are some perennials.
Beans can outgrow the weeds if they are the climbing type and grow up a trellis, in which case they don't need tillage. Or they can be grown on stalks of corn, with squash growing on the ground between the rows. This 3-sisters arrangement is efficient, and provides a fairly good balance of protein to calories, a good balance of amino acids, can build soil fertility, etc.
Designing a system so that it is populated with edible weeds is another option - you can pull them to your heart's desire and then eat 'em.
I think in well done cases, the number should be significantly higher than 2.5 people per farmer. Permaculture won't match the 100 people fed per farmer we see with industrial agriculture, but industrial ag is not sustainable - as you noted, it is like eating oil. Permaculture can compete on yield per acre IMO, but requires more labor (frequent picking on complex terrain, cannot be mechanized), permaculture requires people to live closer to the land.
As for the 4,000 square feet, I was under the impression that an adult male should be able to grow enough food on roughly 1200, while a woman could be as low as 500 square feet. I read all about the break downs in One Circle and I could be slightly off in my interpretation but that seemed to be the gist.
The 4000 includes land needed for growing compost crops. As I understand it, the tiny amounts of land mentioned in One Circle don't include land needed for compost crops. Also, those were pretty darn minimal diets, to say the least!