That is totally dependant on how much data is being backed up with your Daily, weekly and bi-yearly backups. I am assuming that from this
daily backup : 2 weeks : 24 tapes
your Data Protection is set for 2 weeks, and you are allocating 24 tape slots? I would think that, in most cases, that would be sufficient, and the tapes would keep getting reused, but, again, how much data is being backed up
I think that a useful tool for you is the Used media Report. You can either create the report from the Reporting context in the GUI, or, from the command line on your cell manager:
omnirpt -report used_media -timeframe [start] [finish]
The is much more about this command available from the Command Line Interface guide found by clicking Help -> Guides in the GUI