I find most interesting about Seveneves is how the technological assumptions have changed since the book predates the first landing of a falcon 9 booster stage. Getting the tin cans into orbit is described as using traditional use-once rockets. Assuming that you can reuse the first stage 20 times alone would enormously scale up the avaible cargo and m³ you can get into orbit since the upper stages would need not nearly as many rocket engines (and those would be the limiting factor in onstruction really).
if you could get a starship booster to work with similar reliability you could put up about 2-3 times the m³ of the ISS with each launch (assuming a wet-workshop design for he upper stage where the empty fuel tanks are prepared to be made habitable).
assuming an "ISS" can host 10 people that means for 20k people in orbit you need 50 boosters launch 20 times each for a total of 1k launches. This is a very small amount for a larger country like the US on a war economy considering that the upper stages would be the somewhat easier to construct parts.
I would assume that basically every industralized country on earth put up at least as many.
if you could get a starship booster to work with similar reliability you could put up about 2-3 times the m³ of the ISS with each launch (assuming a wet-workshop design for he upper stage where the empty fuel tanks are prepared to be made habitable).
assuming an "ISS" can host 10 people that means for 20k people in orbit you need 50 boosters launch 20 times each for a total of 1k launches. This is a very small amount for a larger country like the US on a war economy considering that the upper stages would be the somewhat easier to construct parts.
I would assume that basically every industralized country on earth put up at least as many.