VMS Help Sys Parameters, SHADOW_MAX_UNIT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    SHADOW_MAX_UNIT specifies the maximum number of shadow sets that
    can exist on a node. The setting must be equal to or greater
    than the number of shadow sets you plan to have on a system.
    Dismounted shadow sets, unused shadow sets, and shadow sets with
    no write bitmaps allocated to them are included in the total.

                                   NOTE

       Review this default carefully. The setting must be equal to
       or greater than the number of shadow sets you plan to have
       on a system. If you attempt to mount more shadow sets than
       the number specified by SHADOW_MAX_UNIT, the MOUNT command
       will fail. Dismounted shadow sets, unused shadow sets, and
       shadow sets with no write bitmaps allocated to them are
       included in the count for SHADOW_MAX_UNIT.

    This system parameter is not dynamic; that is, a reboot is
    required when you change the setting.

    The default setting on OpenVMS Alpha systems is 500; on OpenVMS
    VAX systems, the default is 100. The minimum value is 10, and the
    maximum value is 10,000.