Know how many people are on a video

Know how many people are on a video

So there it is, I want to get a camera video stream from a server, and be
able to know how many people appear on it.
I can install any OS (MAC, Windows, Linux) on my server, use any API (free
or paid) and any language (C++, php etc.) to do so.
I already found some interesting things, such as OpenCV but I think it's
more useful for face recognizing than to know the number of people in a
room...
Any help would be appreciate :)