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

       /BUFFER_COUNT=count

    Specifies a default multibuffer count (0 to 255) for local file
    operations, where count is the number of buffers to be allocated.
    If you use the /SYSTEM qualifier to extend the default value
    systemwide, the maximum default value is 127.

    When you use the /BUFFER_COUNT qualifier, you can use the /DISK,
    /INDEXED, /MAGTAPE, /RELATIVE, /SEQUENTIAL, and /UNIT_RECORD
    qualifiers to specify the types of file for which the default
    is to be applied. If the /BUFFER_COUNT qualifier is specified
    without any of these qualifiers, the /SEQUENTIAL qualifier is
    assumed. If file type is not specified, the default is applied to
    sequential files.

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