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There are 7.5 billion people on the planet.
Some estimate that the earth can sustain 1.5 billion.
By the end of the century the planet may have 10 billion people.
Some estimates show a leveling off or slowing growth in population because of resource scarcity.
Consumption of resources is highly variable. This makes estimates of maximum population also highly variable.
These are very big numbers. And in my mind estimates of this scale are subject to large scale inaccuracies. But one fact that is without doubt is the numbers are large and the problems will be large as well.