If the information is critical to your business, you should have multiple copies. Data that is less essential to the running of your business -- archived customer records, for example -- needs only one copy.
The important question to ask yourself when determining the value of your data is this: If your live data was lost for any reason, what information would you need in order to continue to serve your customers without shutting down your operation for any length of time?
Ultimately, every organization's data is different, but you should have at least one copy of anything if it is worth be backed up in the first place.
That being said, you should test all of the copies: If the tapes aren't being written properly, having 10 copies won't matter because they will all be bad.
Read Pierre Dorion's answer to this question.
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