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A wild guess, with a basis in reality. In order to sustain genetic viability a certain population density is required.

Historians always over estimate the size of armies and underestimate the population. Example in Roman Republican times the Army was four Legions and an equal number of allies, more or less. A legion was about 5000 men between 18 and 45, with a 16 year four campaign obligation, this speaks of a high population density. The Frogs in the 16th century had a standard, more or less, army size of about 25,000 and they could put 3 regional armies in the field, not for long, the Spanish could do the same, again more or less.

In Sargons time the standing army was approximately 3000, as far as we can guess, with levies maybe 15,000 max. When you look at the number of people to support these troops, you can guess about the number of people that support them. For every man on the line it takes 50 to a hundred to support them, 0-80 years old.