VMS Help SET, RMS_DEFAULT, Qualifiers, /BLOCK_COUNT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /BLOCK_COUNT=count

    Specifies a default multiblock count (0 to 127) for record
    I/O operations only, where count is the number of blocks to be
    allocated for each I/O buffer.

    For more information on multiblock count, refer to the
    description of the RAB$B_MBC in the OpenVMS Record Management
    Services Reference Manual.