The backup rule, and the part most people skip
One copy is not a backup and a copy in the same building is only half protection. The principle is old and still correct.
The long-standing rule is three copies of anything that matters, on two different kinds of storage, with one of them somewhere else. The last part is the one skipped, and it is the part that covers fire, flood and theft.
Cloud sync is not automatically a backup. Synchronisation propagates changes, so a deletion or a file damaged by ransomware can propagate too. Services with version history mitigate this, and knowing how far back that history goes is worth checking before relying on it.
An untested backup is an assumption. Restoring a few files occasionally is the only way to know the process works, and it is also how people discover that something important was never included.
For most households the practical shape is an automatic cloud backup plus an external drive kept somewhere else, with phone photographs explicitly included. Photographs are the thing people most regret and most often leave out.
Still have a question?
Describe what you are trying to work out and a member of the team will answer, or direct you to the office that makes the decision.