The Number Problem in the Persian Wars 480-479 BCE

[CaesarAugustus]

in the Parthian campaigns Rome moved armies well over one hundred thousand soldiers (without counting followers), in arid or semi-desert territories.

What evidence do we have for this? As far as I know, the only figures for Parthian/Persian invasion forces are those of Galerius in AD298 (25,000 men) and Julian in AD363 (65,000) - the latter number comes from Zosimus, who quotes much higher figures for tetrarchic civil war armies operating in Europe, for example.

Contrast this with the contemporary panegyricist on Constantine, who said that emperor never led more than 40,000 men, as, in the words of Alexander the Great, a greater number than that is 'no longer an army but a mob'...

Surely the point is that we should be dubious of all our upper-range estimates for ancient armies. The higher the number, the greater the caution.

Already Mark Anthony, according to Plutarch, Anthony's Life - 37, deployed more than 110.000 soldiers, more than 150 years before. 

That we should be dubious is true, but we have to be dubious of everything and not just about upper numbers. But we should avoid to give random numbers. And, as said by the same Roth: "Numbers in any ancient source should neither be accepted nor rejected categorically and the best method for judging numbers is to look at their plausibility, and coherence."

For example, as you may remember, it was simply inconsistent the fact that Septimius Severus lost 50000 men in Scotland.

PS If I rememeber correctly, Zosimus is the one that reported Shapur I conquered Nisibis, despite the fact the the same Shapur I in his same inscriptions at the Ka'ba-ye Zartosht has not included Nisibis in his conquests.