An amusing passage from The Ancient Economy (by M.I. Finley):

Even the rare figure to which an ancient author treats us is suspect a priori .... [W]hen Thudycides (7.27.5) tells us that more than 20,000 slaves escaped from Attica in the final decade of the Peloponnesian War, just what do we in fact know? Did Thucydides have a network or agents stationed along the border between Attica and Boeotia for ten years counting the fugitives as they sneaked across? This is not a frivolous question, given the solemnity with which his statement is repeated in modern books and then used as the basis for calculations and conclusions.