Just to add to the previous comments... take the following situation
5 tape drives (e.g. LTO3 with 400GB uncompressed)
10 clients to backup with a small amount of data on each (say 10GB)
You have set up all 5 drives to backup at the same time and you are not using multiplexing
In this scenario 5 clients will back up to 5 tape drives as the same time. As these finish the remaining 5 will backup to the same tape. You will land up using 5 tape and each will only have 20GB of data.
Take the same scenario above but have 50 clients with the same amount of data. Here 5 clients will backup to 5 drives. The following queued servers will start as the first ones finish untill all complete. Here you will land up using 5 tapes each with 100GB of data.
Basically, it is more about how you run your backups apposed to how much data you are backing up and you will have to find a happy medium between performance and costs depending on your requirements.
You can use the following to restrict the number of media used (you will have to decide based on requirements though)
1. Reduce the number of pools (especially for media being ejected)
2. Reduce the number of concurrent drives that can be used per storage unit
3. Reduce the MPF setting for your volume pool (Maximum Partial Full). In 6.5 and above
4. Remove tapes from the library less frequently. This will allow them to fill up more before being removed.
5. Create Server Groups that allow media sharing between media servers. (6.5 and above)
Other factors that increase your tape usage are the following
1. Multiple retention levels. These can't share the same media by default.
2. Multiple media servers. By default these do not share media.
Cheers
