Let's step back & ask: What can we say in the abstract about the ideal population of the earth?

The ratio of the size of the earth to the size of a human body is about 10^7. (The meter, a human-size unit, was originally defined as 10^{-7} times the quadrant of the earth thru Paris.) That is the order of magnitude of the number of people who, laid end to end, could circle the earth. It is also the order of magnitude of the number who, on rafts in the sea, could all be out of sight of each other (the private-horizon population). We are way past that now.

The square of that number is 10^14. That is the number of people required to cover the earth one layer deep, perhaps leaving a little room to roll over. On a logarithmic scale, at 10^10, we are not quite half way there.

The cube of 10^7 is 10^21. That is the order of magnitude of the population resulting from the conversion of the entire earth into human bodies. There are ethical as well as technical difficulties with such an outcome.