In general, the possible degree of parallelism depends on the following factors:
The number of CPUs available on the database server. Use the following query to find out about the number of CPUs available to an Oracle instance:
select value from v$parameter where name = 'cpu_count';
The setting of the parallel_max_servers initialization parameter. Use the following query to check the current setting:
select value from v$parameter where name = 'parallel_max_servers';
Set this parameter to at least twice the value of the degree of parallelism chosen. For example, to execute a delete statement with a degree of 8, Oracle needs to spawn 16 parallel threads, where 8 threads work on the necessary select to retrieve the rows and 8 perform the delete.