MFA - Tracking Registration & Changes to Setup

There's no such built-in functionality, but you can configure an Alert policy and attach it to the corresponding audit event. Do note that it will fire up for other "Update user" events. More info here: 

https://docs.microsoft.com/en-us/office365/securitycompliance/alert-policies

If you need to scope down the alert to just this specific action, and the device configured, your only option is to write a PowerShell script that periodically reads this data for all users and sends you an email.