If the PARALLEL clause is specified but no degree of parallelism is listed, the object gets the default DOP. Default parallelism uses a formula to determine the DOP based on the system configuration, as in the following:
For a single instance, DOP = PARALLEL_THREADS_PER_CPU x CPU_COUNT
For an Oracle RAC configuration, DOP = PARALLEL_THREADS_PER_CPU x CPU_COUNT x INSTANCE_COUNT
By default, INSTANCE_COUNT is the number of the nodes in the cluster. However, if you have used Oracle RAC services to limit the number of nodes across which a parallel operation can execute, then the number of participating nodes is the number of nodes belonging to that service. For example, on a 4-node Oracle RAC cluster, with each node having 8 CPU cores and no Oracle RAC services, the default DOP would be 2 x 8 x 4 = 64.
The default DOP algorithm is designed to use maximum resources and assumes that the operation finishes faster if it can use more resources. Default parallelism targets the single-user workload. In a multiuser environment, default parallelism is not recommended.
The DOP for a SQL statement can also be limited by Oracle Database Resource Manager.
Oracle Database Administrator's Guide for more information about Oracle Database Resource Manager