成百上千万 is a fixed expression meaning 'tens of millions' or 'millions upon millions'. It is used to emphasize a very large number, often in contexts of population, money, or quantity. The literal breakdown (成百 'hundreds', 上千 'thousands', 万 'ten thousands') builds up to the sense of a huge, uncountable amount. This is not a measure word; it functions as a quantifier or adjective.