I remember doing a research paper on Xerxes' invasion of Greece. At the Battle of Thermopylae, and from memory, contrary to popular belief the Persians are estimated to have numbered between 70,000 (Lowest estimate I found, according to a British Colonel based in Greece during World War 2, if I remember correctly) and 250,000 (highest estimate I found, according to an Turkish historian who's name I've also forgotten). The calculations were made according to the geography of the traversed areas, potable water availability, food supplies and demographics of the Persian Empire by the British General, and by ancient imperial records and accounts by the Turkish historian.

The Greeks also numbered -a lot- more than 300.