To expand on Mr. Sample's comments; there are two limits
- Volume capacity. Until some time in the 1990s, no direct access storage device marketed by IBM had as many as 65535 tracks. This is no longer true, though many 3390 volumes are defined as smaller.
- Data set size. Until z/OS V1R7, the maximum data set size on a single volume was limited to 65535 tracks. This limit still applies to regular PDS data sets, but not to sequential data sets, though allocating a larger data set may be difficult and you must discuss this with your storage management support. Your program may, though this is unlikely, have problems supporting a large sequential data set. We cannot help you.
Of course, regardless of data set or volume size constraints, a sequential disk data set can extend to multiple volumes. The precise rules for this are site specific, and you must
discuss this with your storage management support.
As others have indicated, unless your record size is quite large, I do not think there are any major issues with 25 million records.