Hi Dennis,
If you find a tape library which is supported, then yes, it can have more than 8 drives, but for this type of config you will need a 3rd party software to manage it.
This is the note from NetApp IMT, tape drives sections:
"Netapp builds support and qualifies only tape drives. A tape Library connected via FC, SAS or pSCSI would be considered supported with Data ONTAP if the following conditions are satisfied.
1. The tape library model is supported by the NDMP qualified backup application ( For tape library support contact the backup application vendor )
2. The tape drive within the tape library must be qualified and supported by NetApp. (Please refer to IMT for latest tape drive support matrix)"
Regards,
Radek