In my case, I have a webrct based web app that supports multi-parti video chat and has STUN and TURN servers configured. The connections are done in a mesh way (peer to peer) What happens when some of the users involved in the video chat need to establish the connection via TURN? Do all of the users start to use TURN? What if I'm the user that's behind a NAT? Does that mean that all connections established with me are using TURN?
Connections are peer to peer, so if one connection uses TURN then it doesn't affect other peer to peer connections.
If the user is behind a NAT, he may not need TURN in some cases: all depends on the type of the NAT.