Each license is intended for a single user. A license will allow you to select one device as the video mixer and use its built-in camera. In addition to this device, you can connect up to 8 other devices as sources (9 other devices, if you are running Director Mode on the first device). You only sign in on the device being used as the video mixer.
NOTE: You cannot be logged in on two devices at the same time, or give your login credentials to another person to use on their device. Multiple licenses are required when a user would like to use the video mixing interface on more than one device, or any device that is not owned by the user.