You can connect multiple backup appliances to one backup server. Normally, one backup appliance is deployed per customer, but it is possible to deploy more depending on the scale. This can be managed with Veeam Cloud Connect and the Veeam Service Provider Console (VSPC).
Worker instances and resources will be deployed in the same subscription and resource group as the backup appliance. If you need to have them in the customer subscription, deploy the appliance there, and everything will work as if deployed per individual customer. You can then connect it to Veeam Backup & Replication and Veeam Service Provider Console to fulfill service provider functions.
You can use one Veeam Backup for Microsoft Azure instance to back up more than one subscription in multiple Azure AD tenants. This can be done by adding an account that has access to multiple subscriptions and tenants, or by adding multiple accounts. While this is useful to segment resources, it is still recommended to deploy one backup appliance per customer from a management and scaling perspective.
You can place the backup repository storage account in a subscription separate from both the customer and service provider subscriptions, as long as you have access.