Michael,
If you want to find how many report lines are on a tape, for the active reports, you can run the View SARPAC utility, using the REPORT function (PARM='view_hlq,REPORT'), which will show the total number of lines. If there are mostly text reports, with LRECL=133, that may give you a general idea, but that should include a nominal amount of space reduction due to compaction, to get an idea of how much tape is used.
If you have any further questions, please open a Support case, and we will be happy to address them.
Regards,
Bob Keefe