Oracle Database can process a parallel operation with fewer than the requested number of processes. If all parallel execution servers in the pool are occupied and the maximum number of parallel execution servers has been started, the parallel execution coordinator switches to serial processing.
For information about the PARALLEL_MIN_PERCENT and PARALLEL_MAX_SERVERS initialization parameters, refer to "Tuning General Parameters for Parallel Execution".
Oracle Database Reference for information about using the initialization parameter PARALLEL_MIN_PERCENT