With NetWorker indirectly ...

It is not based on the number of tapes but on the remaining capacity level (based on the volume default capacity of the tape drive type). The attribute is named 'minimum space'. When reached, a notification is triggered.

Otherwise, you can script the following utility ...

  - query mminfo appropriately to get the list of appendable (and recyclable ?) volumes in the jukebox, based on your pool settings

  - count the lines

  - if lower than your trigger level, issue a notification by sending an email or similar

Please check Pg448:

https://support.emc.com/docu9292_NetWorker-7.6-Administration-Guide.pdf?language=en_US

Please mark my answer as correct/helpful answer if it helps.

There is no inbuilt mechanism to do this. you might have to create a script to get this done.