This is exactly the problem. It's not about how many bodies we can stack on top of each other, it's how many people can this one specific area sustainably support, knowing that the number we come up with will fluctuate with time and circumstance and that same calculation has to be made for every "area" we care to define. It's a highly complex problem.

As resources like clean drinking water get more and more scarce, and climate change and the refugees it produces cause all of us to pack into smaller and smaller areas we'll need to start paying more and more attention to this to make sure that we're not destroying what resources we still have.

still could, you need a bigger imagination!

If you cover 50% of the Earth's land with nuclear powered greenhouses, and the other 50% with skyscrapers, you could house and feed a trillion people.

Food is not the limiting factor for population.