Quite a few people interested in ancient warfare know an article by one F. Maurice on the water and roads at the Hellespont. After reading Herodotus’ story that Xerxes marched through the area with 1,700,000 infantry, 80,000 cavalry, and 20,000 charioteers and camel-riders and hiking around the countryside in summer, he argues that an army of 150,000 soldiers, 60,000 noncombatants, and 75,000 animals is the absolute maximum that could have been fed and watered in the area (paragraphs 10, 21, 33). I think this was the Major-General Sir Frederick Barton Maurice who was forced out of the British Army for political reasons in 1918, became a journalist and an advocate for veterans, and died on 1 May 1951. People cite it because he was an experienced staff officer who had walked the ground and talked to classicists like J.A.R. Munro. Its full of details such as that the British Expeditionary Force of 72,000 men, with railroads for supply but just horses for transport, needed 20 square miles for its camp in 1914 (the cavalry were stationed elsewhere and the motor vehicles had not yet arrived). But its certainly not the last say, and while he was talking to British classicists, a retired Bavarian general was preparing a study of the same problem and addressing their arguments.
One Robert von Fischer (d. 1937) commanded the 1st Royal Bavarian Landwehr Division in France from September 1914 to December 1915 and received the honorary title of Bavarian General of Infantry in 1917. He had similar military credentials as Maurice: he commanded a division on the Western Front for 16 months, Maurice served in the Tirah Expedition in Afghanistan, the Boer War, and briefly on the Western Front. And after examining the whole route and the problems involved, he felt that the Persian army was probably no more than 40,000 soldiers strong.