This parameter specifies the maximum number of parallel execution processes and parallel recovery processes for an instance. As demand increases, Oracle Database increases the number of processes from the number created at instance startup up to this value.
The default value might not be sufficient for parallel queries on tables with higher degree of parallelism (DOP) attributes. For users who expect to run queries of higher DOP, set the value to the following:
2 x DOP x NUMBER_OF_CONCURRENT_USERS
For example, setting the value to 64 enables you to run four parallel queries simultaneously, if each query is using two slave sets with a DOP of 8 for each set.
Oracle Database Reference for information about the PARALLEL_MAX_SERVERS initialization parameter