Hi there, Certainly you can use many in affirmative sentences, also in negative sentences and questions. Your example is correct. In questions, people also use any instead of many, they are both correct. Ex: - I have many friends. = I have a lot of friends. - I don't have many books = I have a few of books. - Do you have many friends in your hometown? or Do you have any friends?

It's not a real rule, so much as a matter of "sounding natural." I strongly recommend you practice using the rule you were taught, but once you reach an advanced level, feel free to ignore it. Other things being equal, overuse of many / much actually sounds more formal, and overuse of "a lot of" sounds informal.