So and so writes that 2,000,000 soldiered battled against the Greeks or King Hwatnot had an army of tens of thousands riding horseback across the land.
Really?
How accurate are these estimates? How did pre-Industrial Revolution kingdoms and such amass such large armies of viable and fit soldiers? How did Alexander manage so many troops, plus logistics and tactics and such?
Your post might benefit from a more specific question. Right now, it's very vague and you might not lure in the experts. You are basically asking how accurate sources are and then posing some unrelated follow up questions. The accuracy of sources really varies and is usually properly assessed by historians doing research on a specific case.
I'd advise you to ask for a very specific period or even tailor your questions towards a singular event in order to draw in experts and make it more feasible to answer your question.