How can I create subscriptions for multiple Outlook email accounts?

Question

I am working on an application that may have millions of users in the future. Currently, I have implemented an Azure event hub and Microsoft Graph API to subscribe to the Outlook inbox change event, and the script successfully captures that change event for my test Outlook account. However, the current subscription is focused on a single user account. How can I create subscriptions for multiple email accounts, and what are the best practices and options available for doing this? Thank you in advance.