just going chime in here with an unsourced figure.

quite some time ago I read a few articles claiming a comfortable global carrying capacity of about 500 milion for a post fossil fuel world. this number is old (as I am), and I have no idea if this was misinfo/disinfo/propaganda but if you poke around you may find more detail.

Hardly a study, but some numbers on gas https://www.artberman.com/blog/draining-america-first-the-beginning-of-the-end-for-shale-gas/ and tight oil Eagle Ford https://www.artberman.com/blog/eagle-ford-shale-a-preview-of-permian-decline/ and Permian https://www.artberman.com/blog/beginning-of-the-end-for-the-permian/

Some reasoning on why we are already potentially running critical on diesel https://thehonestsorcerer.substack.com/p/meet-the-gator-growing-energy-demand