What is the per node -2 referring to in Cribl Sizing?

Number of cores minus two

Process count: Indicates the number of Worker Processes to spawn. Positive numbers specify an absolute number of Workers. Negative numbers specify a number of Workers relative to the number of CPUs in the system. like this: {<number of CPUs available> minus <this setting>}. The default is -2.

https://docs.cribl.io/stream/scaling#worker-processes

Note this calculator is meant for sizing guidance, not an exact spec