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

       /MAXMAILBOXES=max-mail

    Specifies the maximum number of mailboxes the processor can
    create in the multiport memory unit. The default is no limit.