Given CO2 levels only really started taking off since WW1 and the world population in 1928 was 2 billion, it would not be unreasonable to set a target at 25% of the current global population, and probably lower, just to stabilise global warming. Nobody is willing to talk about that though. Unfortunately coronavirus is not sufficient. If you want to specifically reduce the human population whilst maintaining and supporting other life forms, you are pretty much restricted to the tried and tested policies of war and genocide, however unpalatable that might be.