Yes but that's not me. Instead, if we skip the biology stuff because our technology there is still in its infancy (CRISPR is utterly amazing, but rather limited, compared to things we can build in the varios engineering disciplines.) And also because I didn't study advanced biology in college.

Okay, so if we take 800 million acres used for animal feed and actual grazing land[], and a heavy approximation of a total 4.8 kwh of energy from the sun per square meter for a single day, and plug in a 1200 calorie diet, we get 110^13, or that the land can support 10 Terahumans, assuming 100% efficient conversion from sun's energy to human calories. Which is ridiculously wrong. (That would be the "impossible to overcome according to the Laws of Thermodynamics and our current understanding of cell biology and chemistry." bit referenced, plus human diet is a bit more complex than pure number of calories.)

So if we can get humans to photosynthesize directly instead of having to eat food (along with the resulting efficiency losses) the Earth can support another few orders of magnitude more humans than there are today by reusing the land that's already being used support cattle. (It's only a small if*. Totally feasible. I only need a few million dollars in funding but we'll have that figured out in a couple years, tops.)

[] https://www.treehugger.com/land-contiguous-us-used-feed-live... [] http://zebu.uoregon.edu/disted/ph162/l4.html