Optimum value for job queue processes parameter

Number of job_queue_processes should usually be "the sum of processes assigned for each batch queue in batch queue configuration” + “5 more processes for Oracle internal jobs”.

Theoretical limit for job_queue_processes as given by Oracle is 4000 processes for Oracle 12c release 2, but as you have mentioned, it could cause performance issues if it is given a high value. 

To get the total number of processes in batch queue configuration, you can execute the following query:

SELECT SUM(process_number) FROM batch_queue;

To set the value for job queue processes, you can use the below command:

ALTER SYSTEM SET job_queue_processes=<value> scope=both;

 
@Chathura Karunarathna has mentioned the basic equation we follow to assign job_queue_processes in a database. The additional 3 Processes at the end of the equation are Reserved for Wiggle Room.

Usually, Oracle Reserved Processes is set to 5, since only very busy batch system will have n=more than five concurrently executing batch jobs.
If job_queue_processes is too low, jobs may be seen waiting in the execution queue for a process to become free.  You can also monitor the job queue and set job_queue_processes to the high watermark of executing jobs.  You can write a monitor to track the maximum count from dba_jobs_running to find the optimal value for job_queue_processes.

I hope this gives a broader picture to the equation mentioned 

Can you specify exactly what falls under OracleReservedProcesses ? A customer is asking for this.