Based on Apple, you can only register up to 100 devices. And you can only remove devices at the beginning of a membership renewal cycle.
However in testflight, you can create up to 10,000 external testers.
AFAIK, you need to register your device in order to create the provisioning profile for you to be able to install the app from testflight on your device.
So question is, do external testers need to have their devices registered, to be able to install the app on their phone? Or is it not necessary to register their devices?
If they need to register their devices, then what good would it do to be able to create 10,000 external testers since only 100 of them can install the app?