成千上百 emphasizes a very large, uncountable quantity. It is less common than 成千上万 (thousands and tens of thousands), but conveys a similar meaning. Both are used to express overwhelming numbers.
mistakes
Do not confuse with 成千上万 (chéngqiān shàngwàn), which means 'thousands and tens of thousands' — a larger scale. 成千上百 is used when the scale is in the hundreds-to-thousands range.
