Initialization parameter JOB_QUEUE_PROCESSES

The initialization parameter JOB_QUEUE_PROCESSES represents the maximum number of job queue processes that can concurrently run on an instance.
However, we should not assume that all job queue processes are available for job execution.

The coordinator process is not started if the initialization parameter JOB_QUEUE_PROCESSES is set to 0.
 The maximum number of processes that can be specified is 1000.

The JOB_QUEUE_PROCESSES initialization parameter is dynamic and it can be modified by an ALTER SYSTEM statement. 

ALTER SYSTEM SET JOB_QUEUE_PROCESSES = 20;