I think that a lot of it has to do with what version of DP you are running
In the original DP licensing scheme, you could have as little as one drive license, no matter how many actual drives you may have had. If you wanted to run drives simultaneously, you would have to have an equal number of licenses, so Sebastian is correct in that respect
You could also used Capacity-based licensing if you have a VTL (Not valid with physical drives) with the understanding that your License had to be large enough to cover ALL of the disk space available for the VTL. For example, you may have a 2TB volume, but only be backing up 50MB. The licensing requirement would be eithr 2 1TB or 1 10TB Advanced Backup to Disk Licenses
As of DP 7.01, this has changed, so that you only have to have sufficient licenses to cover the amount you actually have stored on the Backup device
From the License Autiting Guide (not currently publically available)
Advance Backup to Disk License
Advanced Backup to Disk licenses can be used with Virtual tape Libraries (D2D or VLS) where the
“Virtual tape library - TB based licensing” is checked, and with DP File Libraries.
Advanced Backup to Disk licenses...
Description Part Number Part Number E-Delivery
for 1 TB B7038AA B7038AAE
for 10 TB B7038BA B7038BAE
for 100 TB B7038CA B7038CAE
The Advanced Backup to Disk license is for the space used by protected backups only. Disk space
used for unprotected backups does not need a license.
On DP 6.2 when creating a VLS or D2D device, the quantity of Advanced Backup to Disk licenses
must be entered/assigned to the device.
Until DP 6.2, from within DP, only media management reports of the pools used by the file library are
available. This would give information about the space used, and includes unprotected data.