iCloud question regarding making an archive backup

In iCloud, are archive backups automatic when upgrading to a new version like iOS 11 to iOS 12? I notice sometimes in my iCloud backup folder there are multiple backups for the same phone with different dates on them.

My grandma uses a 5S and I know it gets slow on her and I want to tell her to backup to iCloud and just download the public profile and be done with it. She doesn’t have enough room on her laptop to archive a backup through iTunes.

Basically, if I have an iCloud backup on iOS 11, update to iOS 12 and backup my phone again does that mean I can no longer revert back to iOS 11 now or will there be two different backups in iCloud now?