When query service started you see the following message in query.log
_time=2018-07-18T14:08:45.042-07:00 _level=INFO _msg=cbq-engine started max-concurrency=8 pipeline-cap=512 timeout=0s datastore=http://127.0.0.1:8091 scan-cap=512 pipeline-batch=16 request-cap=256 version=2.0.0-N1QL servicers=32 plus-servicers=128 max-index-api=3 n1ql_feat_cntrl=0 loglevel=INFO request-size-cap=67108864
max-concurrency tells how any logical CPUs that query engine going to use on the machine. For CE this value will be maximum 4, For EE this value is logical CPU’s on the machine.
servicers = 4*max-concurrency -- Number of concurrent unbounded queries processed
plus-servicers= 16*max-concurrency -- Number of concurrent request_plus queries are processed
max_parallelism check out https://developer.couchbase.com/documentation/server/current/settings/query-settings.html
max-parallelism is with in the query. Some operations of queries can be run in parallel (like grouping,aggregation, filter, In EXPLAIN this indicated by “#operator”: “Parallel”) In those situations it will be used that many threads. setting high value will consumes more cpu and memory
Also check this post Couchbase max parallelism