The amount of users that you are being billed for is based off the number of users that you have within your Backupify account.
If you suspend or remove a user from your Google Apps domain the user is not removed from your Backupify account. Rather, that user is marked as 'Archived' in your Backupify/Datto SaaS Protection for Google Apps account. We do not run daily backup jobs for archived users as we cannot pull any data from a user that has been suspended (or deleted) in Google Apps. You can still access the user's backed up data through Backupify.
We will retain archived user backups forever unless you manually remove the user from Backupify/Datto SaaS Protection for Google Apps. You are still charged for archived users and will be unless you explicitly remove them from the Backupify/Datto SaaS Protection for Google Apps application. If you do remove an archived user from Backupify/Datto SaaS Protection for Google Apps, you will no longer be billed for that user, but keep in mind that all backups for that user will be purged.