PARALLEL_DEGREE_POLICY is set to
AUTO, statements that require parallel execution are queued if the number of parallel processes currently in use on the system equals or is greater than
PARALLEL_SERVERS_TARGET. This is not the maximum number of parallel server processes allowed on a system (that is controlled by
PARALLEL_SERVERS_TARGET and parallel statement queuing is used to ensure that each statement that requires parallel execution is allocated the necessary parallel server resources and the system is not flooded with too many parallel server processes.
Oracle Database Reference for information about the
PARALLEL_SERVERS_TARGET initialization parameter