On a finite planet, nothing physical can grow indefinitely. The more of us there are, the fewer resources there are for each of us and for members of other species with which we share the planet.
The challenge of sustainability
The question of human population size is fundamentally one of sustainability. Only resources that can be sustained indefinitely can support humanity in the long term. Given this limitation, we must consider carefully our consumption, what living standards are acceptable, what technologies make best use of the resources available and how to maintain the ecosystems on which we depend. We are already eating into our capital: according to the World Wildlife Fund / Global Footprint Network Living Planet Report, we are collectively consuming the renewable resources of approximately 1.5 Earths, although people in the developed world consume much more than people in developing countries.
Clustered houses
There are no magic numbers — only trade-offs. Any given area of land can sustain many more very low-consuming poor people at bare subsistence level than high-consuming rich people living like millionaires. Population will certainly stop growing at some point. This will either be sooner by fewer births — the humane way of informed individual decision-making on family size — or later by more deaths — the natural, inhumane way of famine, disease and predation or war. There is no third alternative of indefinite growth.
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