There are many variables that determine the number of tape media used. I will list some of them but there are probably other forum members that will add to this list.
1. The number of volume pools used. A tape belongs to only one volume pool. if you use 50 different volume pools then the least amount of tapes that will be used is 50.
2. By default netbackup does not allow sharing different levels of retention on a single tape. if you have 12 different levels of retention in use for the weekly/monthly backups then you will have a minimum of 12 tapes used.
3. If you do not use multiplexing then chances are you will use more tapes. The longer you can keep a tape mounted and in use the more likely you will fill the tape.
4. You will use fewer tapes if you have enabled images to span media.
5. Data compresses differently and you will be able to get more data on one tape than you may in another similiar tape.
6. if you send tapes offsite then you might be sending a tape that is only 1/2 used.
Best practice? Gather metrics for a short period of time and average it out as you continually look for ways to minimize the number of tapes used.
