While this may be in the postgresql.conf it's absence in the docs is made noticeable by the fact both the immediately preceding and subsequent items in the 9.6 docs specify their default values. Furthermore, max_parallel_degree defaults to 2 but the workers come from the pool sized using max_worker_processes - provoking the very question as to how many is 2 relative to the default pool size.
http://www.postgresql.org/docs/devel/static/runtime-config-resource.html#GUC-MAX-PARALLEL-DEGREE David J.
