It is not mandatory to set the PARALLEL query option if the cpu_count is more than 1. But Oracle calculates the default degree of Parallelism as

CPU_COUNT * PARALLEL_THREADS_PER_CPU

Having PARALLEL_ADAPTIVE_MULTI_USER set to true causes the degree of parallelism to be calculated as PARALLEL_THREADS_PER_CPU * CPU_COUNT * (a reduction factor). This mechanism will dynamically size the degree of parallelism based on cpu_count and current work load.

But the run time Parallelism is depends on various parameteres such as

PARALLEL_MAX_SERVERS
PARALLEL_MIN_PERCENT
PARALLEL_AUTOMATIC_TUNING


Regards

Thomas