Perhaps, this is an overly "trivial" question. But, at all the mikva'ot that I have attended, only one man was in the mikvah at a time. Perhaps, this is merely a space issue. But, is there any halacha that prevents more than one perosn being in the mikvah at the same time?
I can't say as to what happens in a women's mikvah, as I haven't been to one while women are there. I assume that the halacha is the same.