It is possible to have one VSA/MA machines per region for multiple Subscriptions, the same VSA can then be used to backup all the subscriptions with different Hypervisor clients in Commvault.
>> for this configuration within Azure you've 2 methods of Authentication we can use to onboard Hypervisor client in CommVault to access the environment:
#1 If all the subscriptions are part of same tenant, we can utilise MSI or system Identity of Access node VMs and add permissions to that VM in all subscription. Follow below doc on how to configure this for one Subscription and same can be repeated for all subscriptions.
https://documentation.commvault.com/2023e/essential/98264_setting_up_managed_identities_for_azure_resources.html
#2 If all Subscriptions are not part of same tenant, or if system identities are not to be utilised one could also use Apps. Create one or multiple Apps for the subscriptions add respective permissions to the application and use the App secret to on-board the Hypervisor in Commvault. Please follow below doc to onboard such for one Subscription which can then be repeated for all Subscriptions.
https://documentation.commvault.com/2023e/essential/31295_setting_up_application_and_tenant_for_azure_resource_manager.html