The number of fault domains for managed availability sets varies by region - either two or three per region. You can confirm the number of fault domains for your target region by running one of the following scripts.
Azure PowerShell:
Get-AzComputeResourceSku | where{$_.ResourceType -eq 'availabilitySets' -and $_.Name -eq 'Aligned'}
Azure CLI:
az vm list-skus --resource-type availabilitySets --query '[?name==`Aligned`].{Location:locationInfo[0].location, MaximumFaultDomainCount:capabilities[0].value}' -o Table