Quick calculus : TLDR : 100+ billions is a hard reach today, but 50+ billions might not impossible There is approximately 63,824,448 km² or liveable land on e

Quick calculus : TLDR : 100+ billions is a hard reach today, but 50+ billions might not impossible There is approximately 63,824,448 km² or liveable land on earth today.[1] Let's cut that in half to account for farms/land lost due to global warming/industries/etc... We get 31,912,224km² of land usable for habitation. Let's assume we cover the remaining with a huge city with the density of Hong Kong (6,690 people/km²). This is for the whole city, not the most populated quarters. We arrive at a population of 213,492,778,560 people, so twice as much as the parent's estimate, with half of the remaining liveable land usable for farms/industries/wildlife parcs/inefficiencies of repartition... Would that be enough ? According to a Sweden study [2], we use 1.6 billion hectares (16,000,000 km²) of farmland today, for approximately 7 billions people. It means that for 200 billions, we need 486,857,142 km². That's a huge increase, and currently the Earth can only sustain 44,000,000km² (same source as earlier, FAO), so an order of magnitude lower. According to a study on the effect of diets on land [3], switching to a vegetarian/vegan (not much change in the result) diet allows to cut the land use by ~4. So we're left with 121,714,285 km² of required land : a bit better, but we still fall way short. That's going to be our actual bottleneck. Let's cut population by 3, and we arrive at ~40,000,000 km² of required land for 66 billions people. It would be a pretty grim dystopia, that's for sure. Most of nature would be erased, there would be a lot of natural catastrophes, and clearly the Earth would be way less welcoming than it is now. But 66 billions people on Earth, given 'sufficient' technological (and most notably political) advancements is not completely out of whack, especially if we manage to produce clean energy (or at least clean on Earth) and to radically improve our crops (with genetic engineering, hydroponic culture, exploitation of oceans maybe ?) Bottom line : this is napkin calculation, a lot of factors have been left out (water to give a main one) [1] : https://www.quora.com/If-every-person-on-earth-was-to-be-given-an-equal-portion-of-inhabitable-land-how-much-land-would-each-person-get https://www.quora.com/If-every-person-on-earth-was-to-be-giv... (I know, Quora is not the most reputable source, but it seems decently sourced) [2] : http://www.fao.org/fileadmin/user_upload/newsroom/docs/en-solaw-facts_1.pdf http://www.fao.org/fileadmin/user_upload/newsroom/docs/en-so... [3] : https://www.elementascience.org/articles/10.12952/journal.elementa.000116/ https://www.elementascience.org/articles/10.12952/journal.el...