3-2-1 rule
3 copies (original, copies 1 and 2)
2 formats (original, tapes)
1 off site (1 tape sent off site)
Its outdated now with all the new tech since tape like replication and storage snapshots.
We just make sure our systems replicate for DR and include snapshots at both sites.
We only use tape for long term archival over 4 months for compliance and business cycles that take over 4 months to discover missing/corrupt/invalid data.