I read a novel a few years ago, which said that a young man (college or high school student) has more than twenty personalities. Can be transformed to deal with every problem.

This novel should be "Almighty Youth King" ("Almighty Youth King" in the series of "Almighty Youth King"). The protagonist in the story has multiple personalities, and each personality has different abilities and skills, including music festival personality, negotiator personality, international language personality and so on. He used these different personalities to solve various problems and challenges, and also faced various tasks and tests. This novel belongs to the category of urban campus and is one of the more popular light novels. I hope this will help you find the answer you want!