Answer
The 'No of Processes' setting directly controls how many concurrent database-level processes that specific queue starts, and therefore how many jobs from that queue can execute simultaneously — setting it to 2, for example, lets that queue run 2 jobs in parallel. Increasing this number, especially for the DEFAULT queue, should only be done after careful analysis, since some jobs need to run in isolation from others, some need to run in a specific sequence, some long-running jobs could otherwise block others behind them in the same queue, and running certain jobs in parallel can create database deadlocks depending on what they touch. Also make sure the database's own job_queue_processes parameter is set high enough to actually support the total number of processes assigned across ALL configured queues combined — as a rule of thumb, job_queue_processes should be roughly the sum of processes assigned across every configured queue, plus about 5 additional processes reserved for Oracle's own internal jobs.