Assume you're on an island with a finite amount of food, now run this experiment again.
The optimal number of humans depends on how finite resources are in an environment. Adding an extra person to an island with a shortage of resources would likely increase human suffering.
That is exactly the sort of assumption that caused Ehrlich to miss the target in The Population Bomb: there turned out not to be a finite amount of food. Or, if the amount is finite, we were "currently" (at the time) producing a tiny subset, as evidence by the vastly-higher quantity of food we're producing now.
As a fellow response comment illustrates, the arrogance of believing you know the "optimal number of humans" means missing out on the amazing and wonderful things future humans are going to make possible.
Assume you're on the island and all n people on the island with you are smashing coconuts all day long just to have enough sustenance.
But maybe person n+1 is able to start catching fish...
I think there's more variables and it's possible our finite resources are more than enough to sustain everyone. However, they may not be enough to sustain endless growth of capital.
Animals have been adjusting their numbers according to availability of resources since the start. Humans found a way out of this but I fear that we are going to fall back into it very soon.
The optimal number of humans depends on how finite resources are in an environment. Adding an extra person to an island with a shortage of resources would likely increase human suffering.