Number of job servers.
Having a large number of job servers running will use more memory and processes, but allows for much faster jobbing of processes because Caché doesn't have to start the processes at the system level and then initialize them.
Job servers are best used when the application creates a significant number of short-lived processes via the Job command. For this type of process where operating system process creation overhead dominates the total cost of running the process, using job servers can be beneficial. If background processes tend to perform extended tasks then there is very little benefit from using job servers.