At this scale, a factor of magnitude or two is of little consequence, and odds are, you can get general buy-in within a "factor of magnitude or two" of the number of humans to live from the vast bulk of people who might have sensible opinions.
In this case, it's actually very easy to get a good estimate: 10 billion is too low, 1 trillion seems a little high, 10 trillion is definitely too high; 100 billion is, as such, really the only good estimate.
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