There is no single answer -- it depends on the importance of your data, its retention requirements, and your confidence in the media. Honestly, if your data is important enough to archive for future use, then it's probably worth making multiple copies. Also, look at using multiple mediums. If you do choose to make multiple copies, be sure to keep your copies in different places. For example, one copy may go to a virtual tape library...
(VTL) as a backup, and another copy may go from VTL to tape. The VTL copy may reside on site, but the tape copy may be moved to offsite storage. Similarly, you may choose to replicate critical files on site, but send backup data to a VTL in another region across a WAN. Spreading out the copies reduces your risk.
Go back to the beginning of the File Archiving FAQ Guide.