I think it is grammatically correct. but maybe, to complete the question, it should go this way: "May I know how many of you there are?" "how many of you" is likely the same asking the question: "how many people share your name" or "how many people are like you?" :)

Hi it's better to say, "Can you tell me, how many of you ....................?" or "Please let me know how many of you.................."