SHADOW_MAX_UNIT specifies the maximum number of shadow sets that
    can exist on a system. 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.

    On Alpha and Integrity servers, the default value for this system
    parameter is 500, which consumes 24 KB of main memory.

    If you do not plan to use Volume Shadowing for OpenVMS, you can
    change the setting to its minimum of 10 (which consumes 480 bytes
    of main memory). Setting the default to its minimum frees up 23.5
    KB of main memory on an OpenVMS Alpha or Integrity servers and
    4.5 KB of main memory on a VAX system. (The maximum value of this
    parameter is 10,000.)

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