Korean does not grammatically distinguish between singular and plural nouns. Thus, while 사람들 means "people", 사람 can mean either "person" or "people", depending on context. 들 is rarely used with nouns denoting inanimate objects. It is more often used with nouns denoting animate objects (people and animals), but then only when it is semantically necessary to make a distinction between singular and plural, or to emphasize plurality. Because 다섯 means plural, 서점에는 다섯 사람들이 있다 is unnatural. And 서점에는 다섯 사람이 있다 is natural 서점에는 많은 사람이 있다 and 서점에는 많은 사람들이 있다 both are correct. 서점에는 많은 사람들이 있다 is emphasizing of plurality.