Dachs said:
I should've said "nobody well-regarded in academia".
True. But several hundred thousands is maybe a realistic number for the size of the Persian army. Also up to 240,000 could serve in the Navy (if the number of 1200 ships claimed by Herodotus is correct - because some other sources claim just 600 ships in the Persian Navy). Assuming 200 crewmen per ship.
The Roman Empire had ca. 435,266 soldiers (including 45,262 in the Navy) in the beginning (most probably) of Diocletian's reign (according to Joannes Lydus Laurentius - his numbers are so precise that they are probably from some official document) and around 645,000 in the early 5th century CE (according to Agathias Myrinae), probably after tetrarchy and reforms of Constantine. However, these are most likely numbers "on paper" rather than in reality according to the book "Amida 359" by Tomasz Szeląg (and also according to A.H.M. Jones) and Szeląg estimates the real number as 450,000 including around 200,000 in the field army and the rest as garrison troops, Navy, etc. ("Amida 359", page 79). Other estimations say 500,000 (C. Zuckermann), 400,000 (A. Cameron).
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Parthian Empire had no standing army (it had a feudal one), but could field ca. 50,000 of excellent cavalry (not including infantry). Sassanids in the 5th - 6th centuries had a standing army of perhaps 70,000 regulars (K. Farrokh), not including numerous support units, levy infantry units, maybe some cavalry was not among the regulars as well (I'm not sure if all of nobility served as regulars or rather some of them served as feudal troops typical for the Parthian cavalry).
Majority of the 70,000 Sassanid regulars were cavalry, but also some elite spear & shield armoured infantry and elite foot archers were among them.
The Han Empire was also able to field large cavalry forces for their major campaigns. For example in 126 BC against Xiongnu they fielded over 100,000 cavalry, similar number was fielded in another campaign in 119 BC - two cavalry armies 50,000 strong each (in total also 100,000 cavalry).
Apart from cavalry, they were also capable of mobilizing enormous numbers of chariots if needed. For example "The Military Storehouse of YongShi's 4th year Equipment Account Book", which lists equipment stored just in Donghai Commandery, mentions 7174 chariots of 22 different types.
The Han Empire developed an organized system of breeding good quality horses on large scale.
Of course Ancient China was also capable of mobilizing a much bigger number of infantry if needed (surely at least several hundred thousands).
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The Ptolemaic Empire in ca. 246 BC fielded according to royal registeries (basilikai anagraphai): 300 war elephants, 2000 chariots, 40 thousand cavalry, 200 thousand infantry, enough weapons stored to equip further 300 thousands recruits if needed and considerable Navy (this is according to N.G.L. Hammond).
In 242 BC Carthage mustered a fleet numbering 250
Pentera ships. Each had a crew of 300 rowers and could also transport 200 soldiers. According to B. Nowaczyk, Carthage between 5th and 3rd centuries had a fleet ranging in size from ca. 200 to ca. 400 war ships and ca. 1000 transport ships.
Total Naval casualties during the 1st Punic War (264 - 241 BC) amounted to ca. 700 (Rome) and ca. 500 (Carthage) war ships sunken.
The Ptolemaic fleet at its peak strength (around 260 - 250 BC) amounted to (it seems slightly exaggerated though) 4000 ships - 2000 small ships, 1500 war ships of various kinds (the largest of them being
Pentera ships) and 800 yachts. Later the Ptolemaic Kingdom developed a new kind of war ship.
The Ptolemaic fleet had at least 90,000 of rowers alone.
This new kind of war ship was enormous
Tessarakontera - its crew numbered 400 sailors, 4000 rowers and it could transport 3000 soldiers.