Do I Have To Buy A License For Each Device That I Use?
Switcher Basic - You can use a single license for a 4 camera setup. The license is tied to the Apple ID used when installing the app, so each device will need to be linked to the same Apple ID. Keep in mind that Apple only lets you share an app on up to 5 devices using your Apple ID (details). Once you have reached this limit you cannot reassign your license to another device without disabling it on all devices.
Switcher Studio - 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 3 other devices as sources (4 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. Devices used as cameras/sources do not require login.
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.