The structure 何 + Counter is used to form questions about quantities or amounts.
人 (にん): People
枚 (まい): Flat objects
冊 (さつ): Books
個 (こ): Small objects
台 (だい): Machines or vehicles
本 (ほん): Long, cylindrical objects
時 (じ): Time (hours)
円 (えん): Money (yen)