So far as the English goes, in the sentence, "How many of you are there," the word "there" has a different meaning from its other use as the opposite of "here." When I say "you are there," I mean, "you are not here." But when I say, "How many of you are there?" I mean, "how many of you are present." It's confusing because the meaning is more like "how many of you are here," but it is simply not said that way. I would only say "how many of you are here?" if it were a specific reference to THIS location as opposed to some other location. To determine a quantity of people, "How many of you are there?" is the custom.