I was studying Chinese grammar from a textbook. Somewhere in the book, it explains the difference between 几 and 多少. The book says that 几 needs a classifier. But using a classifier with 多少 is optional. It also gives some examples, like 多少（张）照片，多少（个）朋友，多少（个）系 But when it reaches 人, it writes: 多少人 not 多少（个）人
I wonder if 多少个人 is incorrect or obsolete?