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.
There are four people in my family.
How many people can fit in this room?
五人分の料理を作った。
I cooked enough food for five people.
How many people are coming to the party?
人 is the general-purpose counter for people, while 騎 is reserved specifically for mounted riders such as cavalry.
人（にん）is the neutral counter for people, while 名（めい）is a more formal, polite counter often used in official announcements or service contexts (三名様).
人（にん）is the counter attached after numbers (三人), while 人（ひと）is the ordinary noun meaning 'a person' and is read differently.
Standard kanji form for the counter にん.
The counter にん is derived from the on'yomi of the kanji 人, which means 'person'.