Switching to nationhood means additional stability for every civ you're at war with, and switching to occupation means additional stability for every city you capture. That's if you have enough stability to endure a few turns of anarchy.
According two different people of that era, the numbers were.
Greek Total
5,200+ (Herodotus)
7,400+ (Diodorus)
11,200 (Pausanias)
Persian Total:
70,000–300,000 (modern estimates)