I've been able to get this sample custom policy working on my B2C tenant. However, once a user signs-up with the authenticator app the first time, there doesn't seem to be a way to add a new device (or delete their existing device to re-register).
How do I change the code to allow for adding additional devices.
As the code stands, you can't.
This is because the code uses "numberOfAvailableDevices" = 0 to allow you to sign up and "numberOfAvailableDevices" > 0 to ask for the code.
You need to copy the policy and make another policy just for further sign-up and then reference that somehow in your app or elsewhere. This policy won't check for the number of devices.
You could use this policy as a guide to add another button/link to the page to do this. It adds forgotten password as a button/link and then sets a flag to show this option was selected.
To delete a device, you need this API.