Each instance runs on its own thread. The default number of instances is set to 1.5 times the number of cores the PC has, in order to maximize core/thread usage. Those values can be adjusted (for instance, if you are running mostly small jobs and you have a lot of RAM available, you could have 2 or 2.5 instances per core).
It's pretty much empirical how you can tweak those values, but note that at some point, overtweaking will result in degradation of performance, so monitor your logs closely to determine when you've hit that threshold.
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Technical Product Manager
I don't want to achieve immortality through my work; I want to achieve immortality through not dying - Woody Allen