What humanity needs to know is the maximum number of humans that is indefinitely sustainable. At the US level of energy usage, that level is probably under one

What humanity needs to know is the maximum number of humans that is indefinitely sustainable. At the US level of energy usage, that level is probably under one billion. At a European level of energy usage, we'd probably sustain about a billion and a half. The current population level of just under 8 billion people is absolutely unsustainable, even at African Nomad levels of energy usage.