A sequential data set can have 16 extents on each volume.
An extended-format sequential data set can have 123 extents per volume.
A PDS can have 16 extents.
A direct data set can have 16 extents on each volume.
A non-system-managed VSAM data set can have up to 255 extents per
component. System-managed VSAM data sets can have this limit removed if the associated data class has extent constraint removal specified.
A striped VSAM data set that is not system-managed can have up to 255
extents per stripe. Striped VSAM data sets that are system-managed can
have this limit removed if the associated data class has extent constraint removal specified.
A PDSE can have 123 extents.
An HFS data set can have 123 extents on each volume
But keep in mind that an extent is not always equal to the primary or 2ndary values specified in the DD card for the dataset.
A primary alloc may take up to 5 extents to be satisfied. That will leave only 10 extents to satisfy your 2ndary allocs.
The 2ndary allocs may take up to 3 extents to satisfy each alloc. So that means you may get as little as 3 2ndary allocs.
The reason for this is that each alloc is attempted to be satisfied w/1 extent of contiguous DA space. If it can't, it takes what it can get contiguously then uses another extent for the remaining space, if it can't get enough contiguously, it tries again until it uses all the extents and ... well you get the idea.