Recently updated research reports:
We at NPG believe that it is in the range of 1.5 to 2 billion, based on the assessments of the major population and resource scientists most concerned with the limits of economic growth. That was the level of world population as recently as the first decade of the 20th century, before the pressure of numbers had generated the environmental damage now visible.
Is this achievable? Perhaps, if the entire world limited their replacement ratio:
But a massive reduction in the size of world population will require a massive reduction in the population of every country in the world, developed and developing alike. That in turn will require the achievement of fertility rates well below the replacement level of roughly 2.1 children per woman. Such levels have already been reached by most countries in Europe, some of which are already experiencing population decline. If almost no women had more than two children, the world’s fertility rate would drop well below the replacement level because some women choose to have only one child, or remain childless. For at least the next two generations the two-child maximum family must become the world’s norm.