Library /sys$common/syshlp/sysgen.hlb SHARE, /INITIALIZE, Qualifiers, /MAILBOXES  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /MAILBOXES=mail

    Specifies the total number of mailboxes permitted in the
    multiport memory unit. The mail value is an integer with a
    default of 32.