You don't need to read up the military history of each and every Ancient empire. A single text on (pre-railroad!) military logistics would do to get an idea what was needed to move around and supply some 30,000 men, leaving a healthy doubt when anything larger than 50,000 men is stated in the sources, and rendering any numbers of more than 100,000 in a single army as "completly impossible", and some 1,000,000 a joke at best.