The fault domain in the Azure Availability set describes the VMs in the availability set that share the common Power, Storage, network switch, etc. If there is a failure in the fault domain then all the resources in the fault domain will become unavailable.
Before running the command make sure that you are connected to the Azure Cloud (if not use ConnectAzAccount) and proper azure subscription (Set-AzContext to set the Azure subscription)
$availablityset = Get-AzAvailabilitySet -ResourceGroupName ResourceGroupName -Name AvailabilitySetName
To get the Fault domain countm