Honestly I just flipped the right bits in the aws load balancer (maintain persistent connections, just the first thing you are told to do when googling aws load balancers and web sockets) and setup the instance scaler to trigger based upon "# open connections / num servers > threshold".

Ideally it is based on the rate of incoming connections, but so long as you leave enough headroom when doing the stupid simple scaling rule you should be fine. Just ensure new instances don't take too long to start up.