In sci.space.policy Ian Stirling wrote:
In sci.space.policy Gactimus wrote:
How many people can the earth support?
snip
This naive calculation indicates that you need around 150 tonnes of crust
to build a human, or around 300 tonnes for an american
Assuming that the crust goes down 10Km, and has a density of 4, that's
around 270 people per square meter of crust, that's about 4*10^17 people,
or the equivalent of the population of the UK, for every person alive now,
neglecting the earths core.

If you want a conventional biosphere feeding them, rather than direct
energy-food converters of some sort, you'r probably going to need
a fair bit more nitrogen, which might limit you to 4*10^15 people or so.

Making possibly incorrect assumptions, you can probably scrape up
4 or 5 earths worth of rocky matter in the solar system.
Assuming that these have some shortfalls, equivalent to needing 1500 tonnes
per person of mass, and using the whole of their mass takes the "people only"
figure to around 10^20.

Assuming it's not possible to get at gas-giant cores, but is possible to
mine their atmospheres for the needed N/C/H, it seems reasonable that
the first bottleneck is phosphourus.
This would take the "people only" totals up to around 10^22, or 10^23.

You can maybe get another order of magnitude or two, by synthesising
Cl and P, but for much more you have to dismantle the gas giants to get
at their cores.
Maybe another couple of orders of magnitude.
So, 10^26 people might be a reasonable guess at an upper limit, or 10^24
eating a largish fraction of naturally grown food.

10^26 people is about one human for each one of every current humans
intestinal bacteria.

To get any more, you need to start on either mining the sun, energy-matter
conversion, or redefining what you mean by a human.