What is the ideal world population . . . that is, of humans? One quick-fire response is “Ideal for what?” All right, ideal for approximately what we are already doing.
Like all species, we had to have the ability to produce slightly more offspring than we strictly needed to in order to survive because if we had produced slightly fewer than replacement numbers then we would have faced extinction a long time ago.
The result, as in all species which overpopulate naturally sooner or later, is that there is far from sufficient food to feed to give us all a nutritious diet. We even experience regional pockets of extreme starvation. These will become larger and more numerous as we reach what is expected to be turn-around period in about 50-70 years’ time when the human population finally starts to decline.
So what’s the figure to be? The answer is that we won’t decide. The environment will take that decision according to circumstances. In every generation new mutations arise. Some are benign but some are such serious handicaps that those males tend not be be selected by young females as desirable partners and parents. It’s the balance between benign mutations and the sub-par ones that decides whether the population goes up or down — given enough food.