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

    MAXPROCESSCNT sets the number of process entry slots allocated at
    bootstrap time. One slot is required for each concurrent process
    on the system. Each slot requires 6 bytes of permanently resident
    memory.

    The default value is normally configured to allow you to create
    the desired number of processes. If the following message
    appears, you need to increase the value of MAXPROCESSCNT:

    %SYSTEM-F-NOSLOT,  No PCB to create process

    MAXPROCESSCNT has the AUTOGEN, FEEDBACK, GEN, and MAJOR
    attributes.