Oracle has background processes and server processes.
In a dedicated mode, each user process uses server process to talk to oracle,
means one to one correspondence.This server process remains active during the life of the session.v$license view gives you the HWM of the sessions connected concurrently,I generally use BGP+HWM+50.
In parallel processing check the init paramter and dynamic performance views and add these one to the above formula.
In MTS check its a bit complicated, but use the dynamic performance views and the init setting to find out your requirment.