Q. Is it true the number of fault domains available varies by region when using managed disks?
A. Yes. While normally with ARM the default is 3 fault domains, when using managed disks which improves the reliability of the Availability Set by also ensure the disks used by the VMs are also isolated, the number of fault domains varies between 2 and 3. The exact maximum number is documented at https://docs.microsoft.com/en-us/azure/virtual-machines/windows/manage-availability.