When you see this on a number, for example ７人 or ３人, you're using it to count people. Just remember, both 一人 and 二人 have alternative readings. From 3 and up it's just the number plus にん, like this. ５人 is ごにん, １０人 is じゅうにん, etc. This is the counter for people, which makes this word the number of people.