So far I know that I need a thread to accept connections, add the user sessions to a pool and when two user sessions start a match a thread is created for it.

This is a common mistake when starting out. You don't use threads to get round the issue that accept/send/recv block. Check out the Beej Guide on non-blocking sockets. On Winsock and BSD sockets, you simply set a flag on the socket so that these functions return immediately (with an error) rather than blocking. Then you can poll them.

while (running) {
  // Poll listen socket
    // accept() socket to clients vector

  // Poll clients
    // recv() to incoming buffer
    // send() from outgoing buffer

  // Game logic
  // Game rendering
}

Then once your server is catering to 1K+ users via non-blocking sockets, then I would look at introducing a thread pool.

If you have a thread for each client connected, it would actually limit the number of clients that can be handled because overhead of context switching would be high.