The way our TURN billing is structured is that we do not count the number of streams going through the TURN server at all. We count the data crossing the network interface corresponding to your App Keys. Bandwidth is measured as the total sum of your TURN server-relayed traffic.
Also, when TURN support is enabled for a given key, it will only take effect when required. So users able to connect successfully without TURN will not be metered. Only users requiring Media Relay Services (TURN or MCU/SFU) will be subject to bandwidth metering. Billing is per GB, charged by byte rounded to the nearest cent.
Please see here for further bandwidth pricing details.