counter
Used after numbers to count people. Note that one person is ひとり and two people is ふたり, while three and above use にん (e.g., さんにん).
教室に学生が三人います。
There are three students in the classroom.
この部屋には何人入れますか。
How many people can fit in this room?
The noun meaning 'person', not a counter.
The counter にん is derived from the on'yomi of the kanji 人, which means 'person'.