I don't know how good NetBackup Operations Manager is for capacity planning but there are a few 3rd party tools that do a good job of this.  The one I use is Aptare StorageConsole and it's helped us justify upgrades several times in the past.  I've been trying to get Aptare to provide better reports for volume by media server but I haven't found a great solution yet.  The data is in the database, and they give you the hooks to write custom queries, but I haven't had the time to write the proper query to summarize volume by media server.

For managing the quantity of tapes, we use an open source solution - MRTG graphs the scratch tapes vs open library slots on a regular basis. We then watch this over time to see if the scratch pool starts to get low or if we're out of slots in a library.  You could easily script this yourself and throw it into a spreadsheet for your own graphing.