The O365 Total Backup Subscription is the current plan offered that will allow you to backup your O365 tenants. The Total Protection Prepaid plan is for End Users only, if you are looking for Subscription billing for MSPs, please see here.

Here you can find more details on how licensing works:

• this plan will consume the number of prepaid seats available if users are enabled for backup and hold a valid M365 license that has functionality for the protected M365 services (emails, files [OneDrive/SharePoint], or Teams)
  
  
• if a user does not hold a valid M365 license, it will not consume a seat and will retain any backup data for free
  
  
• an O365 shared mailbox (unlicensed) is free
  
  

Scenario to explain licensing:

• my tenant has 5 M365 licensed users and I purchased a Total Backup prepaid license for 5 seats. I enabled 5 users for backup.
  
  At this stage, 5/5 seats are consumed from my prepaid license.
  
  
• I then removed an M365 license from 1 of my users, which means I have 4 M365 licensed users. I do not un-provision the backups for this user, so the previously taken backups are kept.
  
  At this stage, 4/5 seats are consumed from my prepaid license.
  
  
• I then add a new M365 licensed user to my tenant and I enable this new user for backups.
  
  At this stage, 5/5 seats are consumed from my prepaid license.
  
  
• The previously M365 unlicensed user has now been re-licensed (which was never un-provisioned for backup). This will now occupy 6/5 available seats from your prepaid license which will be considered as a breach of license and will suspend all backups. Backup data will be retained, however, you will need to free up the appropriate number of seats or upgrade the license accordingly.