成百 is a fixed expression meaning 'hundreds of', used to indicate a large but approximate number. It is most commonly seen in the four-character phrase 成百上千 (hundreds upon thousands), but can also appear alone before a noun to mean 'hundreds of'. Unlike the standalone number 百 (one hundred), 成百 emphasizes a range — 'many hundreds' — and is often paired with 上千 for rhetorical effect. You'll encounter it in formal or literary contexts, not in everyday counting.