In other words 100 billion to 1 trillion. The world’s land area, excluding Antarctica, is about 50 million sq. miles. With a comfortable population density of 1000 ppl/sqm (like in South Korea, Holland, Israel, or the state of New Jersey) you would get to 50 billion (same as Trantor). The population density of Singapore is close to 20k ppl/sqm, that would give you 1 trillion.
If you are willing to pad the Earth with buildings 16 km high, you could pack at least 1000 times more people, more likely 1 million times more.
I don't remember the number. What I recall is just that it was an absurdly huge estimate. And I'd love to have a copy of that ~1975 letter, if anyone has access to requisite tools.
And, in case it's not obvious, it's not a world that I'd care to live in.
