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PARALLEL_MAX_SERVERS 
It specifies the maximum number of parallel execution processes and parallel recovery processes for an instance. As demand increases, Oracle increases the number of processes from the number created at instance startup up to this value.It is derived from the values of CPU_COUNT, PARALLEL_AUTOMATIC_TUNING,and PARALLEL_ADAPTIVE_MULTI_USER
If you set this parameter too low, some queries may not have a parallel execution process available to them during query processing. If you set it too high, memory resource shortages may occur during peak periods, which can degrade performance.

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parallel_automatic_tuning 
When PARALLEL_AUTOMATIC_TUNING is set to true, Oracle determines the default values for parameters that control parallel execution. In addition to setting this parameter, you must specify the PARALLEL clause for the target tables in the system. Oracle then tunes all subsequent parallel operations automatically. As part of the automatic tuning, Oracle will enable the PARALLEL_ADAPTIVE_MULTI_USER parameter. You can override any of the system-provided defaults if desired


Hope this helps you