Some of the established demonstration permaculture farms feed one person on 5 acres. (The upper limit might be 4 acres in some locations.)

There are about 22.5 B acres of land on earth available for cultivation and agriculture.

That could feed about 2.5 B people if everything is totally optimal. This would be no draft animals, no digging, no tilling, like a giant designed food system that needs less energy to maintain, more passively regenerated.

Its not that different of a carrying capacity, I’m just tossing this out there as an alternative approach to the calculation.

Basically let’s get rid of 500 million draft animals and have humans instead.