How you are calling endpoint and what version of CB
In CE each query service will use 4 cores and 4*4 = 16 concurrent queries , but can queue 4*256.
Unless you use SDK you need to distribute the requests each query service.
Above error means you don’t have ports open check the all required ports are open. Error: An unknown N1QL error occured. This is usually related to an out-of-memory condition