3-2-1 backup rule

The original plus two backups covers the case where one copy fails while you are restoring from another. Two media types covers a fault that takes out a whole class of device at once, such as a bad batch of drives or a controller failure. The off-site copy covers the room burning down or being burgled.

Two beliefs break it. RAID is redundancy against a drive dying, and it happily mirrors a deletion or a ransomware encryption to every disc in the array. A synced cloud folder has the same hole, because deleting the local file deletes the remote one. A backup you have never restored from is a guess.