Release : 12.6
Component : CA VTAPE
Each record in the Global VCAT and BSDS is 4 KBs in size. 7298 of those records are reserved for control information like the Recall Queues, the Backstore Queues, etc. The rest of the records are for Virtual VOLSERs, one per record.
If you have a data set with 7450 tracks. There are 12 4 KB records per track, so you have 7450 x 12 or 89,400 records. 89400 - 7298 = 82102 records for Virtual VOLSERs.
However, these files are initialized and only the initialized space can be used. If you allocated the file at 7450 tracks, but only initialized it to 2000 tracks, then 2000 x 12 = 24000 records. 24000 - 7298 = 16702 records for Virtual VOLSERs.
How much space is actually intialized? These are VSAM files so you do an IDCAMS LISTCAT and look for the High Allocated and High Used RBA values. High Allocate RBA or HARBA is how much space you have allocated. High Used RBA or HURBA is how much of that space is currently populated or initialized.
Take the HURBA value and divide it by 4096 to get the number of initialized records in your Global VCAT and BSDS.
Subtract the 7298 control records to determine how many records you have for Virtual VOLSERs.