VMS Help V73 Features, System Management Features, New System 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.

    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.

    Note that this parameter does not affect the naming of shadow
    sets. For example, with the default value of 100, a device name
    such as DSA999 is still valid.