Zero or one backups depending on the type of data generally.
For a system if it can't be redeployed from a list of requirements then I Foundationally ducked up. It means either I'm depending on something fragile, unknowable or over complicated.
Raw data on the other hand goes on a raid array. If I need more redundancy then that, then a single extra off site backup on a cd or something.
A back up on the same drive as the original isn't a backup. It's a waste of space, and a stupid idea.
A system that requires excessive nonsense is not reliable.
If I have a on and off site back up and both fail. Then either the world is literally fucking on fire and it doesn't matter. Or someone dropped a bomb on my storage.
Either way I have bigger problems at that point.