I think we can estimate an upper bound pretty easy. North America for instance, particularly the USA currently produces enough food, water, and oil to fuel its whole society. It also has the ability to do all the manufacturing needed for modern society even if some of it is currently run cheaper in other countries. Outside of a few resources that are more easily acquired elsewhere it could be a pretty self contained country. It already produces more than half of the total research papers and R&D in the world.

You could easily eliminate the rest of the worlds population and still maintain the same standard of living with a few adjustments. So I’d put the upper bound at the current US population of about 300,000,000 people.

Now could you go lower than that? Probably. You wouldn’t need anyone over the age of 70 to keep the country going, so kill everyone over 70 and reduce that number by 60 million to 240 million.

You obviously need workers and children so no chance for reduction there.

The bottom 10% ish of the population is likely a net drain so eliminate another 40 million.

That puts us at 200,000,000 people left without a huge loss of function.

I don’t think you could go much lower without impacting luxury high end living.

So I’ll say 200,000,000 is about as low as you could go.