Create a parameter for number of workers in cluster mode

[gusostow]

[H Zhang]

if (cluster.isMaster) {
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork()
  }
  
  The number of workers in your cluster is bound by the amount of CPU your server/computer has. It's probably not wise to go beyond the CPU limit as I imagine its probably one thread per CPU. 

[gusostow]

My desire was to use fewer than my total CPUs

[H Zhang]

You can edit the file to use less than the total amount if CPUs and pass it in as a parameter.