InQuizitive Editing the Errors that Matter Practice Test

Counting people uses a count noun. When you want to express how many individuals there are, you use the word number after the definite article to refer to a countable quantity. Amount is reserved for things you measure that aren’t easily counted (like water or sand). Quantity can be used for a measurable amount, but with people it sounds less natural, and you’d more commonly hear “the total number of people.” So the natural, idiomatic choice here is number.