你好！ 大勢 is a noun. It means "trend, great number/crowd of people". So an example would be そこには大勢の人がいた。There has been a crowd of people.

when we use 大勢 word, 大勢の観光客がいるね！(There are so many tourists.)今日のコンサート、大勢いたね！(It was so many people in this concert today.) so 大勢＝たくさん. 大勢 is more polite word, use it when there are so many people. hope you understand with this answer.... :)