• The parameters SESSIONS and PROCESSES determine the size of two arrays in the SGA.

• If you try to create more sessions on an instance than specified
  by the SESSIONS parameter, you will get an ORA-00018: maximum
  number of sessions exceeded.

• If you try to create more processes on an instances than specified by the PROCESSES prameter,
  you wil get an ORA-00020: maximum number of processes exceeded.

• The number of sessions and processes on your instance can been seen using a COUNT(*) against V$PROCESS and V$SESSION.

Your job is to determine resonable values, so your users can continue to work without getting neither ORA-18 nor ORA-20 during normal conditions.
(A run away job, that keeps starting new sessions is not a normal condition, and it should be stopped by ORA-18 or ORA-20 whichever happens first).

• You can join the two V$-views using the columns V$SESSION.PADDR and V$PROCESS.ADDR.

• It is quite normal to see a 1:1 relationship between processes and sessions.
  However one OCI-program can create multiple sessions belonging to one process.
  Users of Oracle Portal will experience this behaviour.

• The default value for SESSIONS is 1.1*PROCESSES + 5.
  It is appropriate for some databases and inappropriate for others (e.g. Oracle Portal).

Also, I remember a limit on the OS regarding the number of processes
allowed per Oracle user - also I know there is a semaphore
relationship.

• On HP-UX there is a kernel parameter called NPROC.
  Like PROCESSES and SESSIONS it should have a reasonable value.