Virtual machines inside the Availability set get the update automatically, they reboot together and they are used for the patching of the virtual machines.
To get the update domain count from the availability set, we can use the below command.
Before running this 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)
To get the availability set,
$availablityset = Get-AzAvailabilitySet -ResourceGroupName Resourcegroupname -Name AvailabilitySetName
To get the update domain count,