Assume such a situation: One video show or conference room has 1~5 publishers and 500~1000 subscribers. Subscribers watch publisher's video. Publisher's video and audio is captured in real time.
So I wonder, does this system(open source) exist? if not exist, how to implement?
There are many solutions for your need.
If you doesn't want to host the solution yes one of them is Youtube Live, or Twitch if you want to publish a game.
But you can also host a Media server like MonaServer, FMS, Wowza, Red5... I work on the first one, it has P2P, it is free and open source so contact me if you are interested and want some help.