Hi !
There is no thumb rule on how the PROCESS and SESSION is to be related, however there exists an relationship between them.

For every session, a process is created. It all depends on the number of concurrent sessions.

I don't see a 100 process instance handling 1000 concurrent sessions...as is the case....when ever a new session is created a process gets added up. If the number of process execeeds the PROCESSES parameter oracle throws an error.

A simple calculation of the no. of PROCESSES
= No. f concurrent sessions + 5

Where the 5 additional processes are for SMON, PMON, DBWR,LGWR...etc.,

Cheers,
Nanda.