So, during the battle of Cannae, Hannibal's forces (50k men) captured around 15k Roman soldiers. Looking at other battles in different time periods, the numbers of captives are staggeringly high. How would they be fed and wouldn't they put strain on supply lines? How would they be captured in the first place? What would be done to them in the end? And, most importantly, how do you keep and control a group of hostile captives big enough to populate a small town, in enemy territory, whilst on the move?
Disclaimer: posting this again due to getting no answers on the first one. I wanna know, damn it :D
For the classical period, the first question we need to ask is not 'how did they take so many prisoners?', it's 'did they take that many prisoners?'
Estimates of the sizes of ancient armies, camp followers and prisoner pools are a fairly controversial area of discussion among historians.
There's a great post in this thread about how Herodotus's estimates, as an example, are largely regarded as implausible despite following and explaining his own methodology.