Veeam Backup for Microsoft Azure allows you to add only one Microsoft Azure account. After you add an Azure account, you will get access to subscriptions that belong to this account only, that is - subscriptions that belong to other accounts will not be available.
Veeam Backup for Microsoft Azure allows you to add only one Microsoft Azure account as a service account.
To see other subscriptions, use Azure Lighthouse. Azure Lighthouse allows service providers to manage resources of their customers.
When you add an Azure account, you can choose an Azure Active Directory Group to which you can add a security AD application and that contains customers resources that you can back up using Azure Lighthouse.
For more information and a step-by-step configuration, see the Azure Service Account section of the Veeam Backup for Microsoft Azure User Guide.