Chromecast Device Registration and Concurrent Stream Limit
The sender application and receiver application are different devices. If you play content on both the sender and receiver devices, the mobile device and Chromecast will both be registered as devices. This typically happens on a 'play and cast' scenario. If the user does a 'cast and play', which means playback only happens on the Chromecast receiver, then only Chromecast gets registered as a device.
However, for concurrent stream limits, play from a sender and a receiver application is counted as a single concurrent stream. An example of this is if the user does the 'play and cast' scenario from their mobile device, where they play the content on their mobile device, use their mobile device to cast the content to a receiver application, and then continue to watch the content on the receiver. This is counted as one concurrent stream. The same count happens on a 'play and cast' scenario (one concurrent stream counted for play from a sender and a receiver).