The 3-2-1 rule is simple: 3 copies, 2 storage types, 1 off-site.
Why one backup is not enough
A backup on the same disk disappears with the disk. Modern ransomware encrypts reachable backups too.
The practical setup
Daily local copy, automatic S3 replication and a monthly restore test.
A backup never tested through a restore is not a backup. It is a hope.