This doesn't really translate well into English. I assume it means "How many of you are there in the class?", but I don't know if that's an interpretation issue or if this sentence doesn't have a proper equivalent in English.
Yes, the sentence in English should be "How many of you are there in the class?" which is an accepted answer. In Danish, where we would say "There is X of us/you", you would say "Vi/I er X"
How many of you in the class, should be accepted too. I suppose that technically it is wrong because there is no verb.
'i klassen' should mean in the classroom, or in the class, i.e in 'grade 2'?