"How many are you in the class?"
Translation:Hvor mange er I i klassen?
The sentence is incorrect English. This is one of the most common mistakes that European learners make, even those who speak English at a very high level.
The correct sentence would be "How many of you are there in the class?" (to which you might respond "There are twenty of us in the class.") In most European languages, such as French, German, and Danish, you use the verb to be with a pronoun to indicate how many people there are, but in English, you have to use there are with a pronoun.