Dividing the Lands (Part-3)
The 2 million tons of grain that Tibias produced was large enough to sustain 7 to 8 million people.
Alexander got this answer by assuming an average citizen would need on average 200 kg of wheat annually (less than 1lb a day) to properly sustain himself.
This was an average number, taken as an aggregate of the population, i.e. men, women, and children combined, with one man usually being counted as being equivalent to a pair of woman and child.
This method yielded a population of 10 million, but it had to be also remembered that farmers needed to keep a quarter to even a third (25% to 33%) of their produce as seeds for replanting next year.
Thus Alexander's more reasonable answer of 7 to 8 million.