Limit the number of workers that can be spawned

[ryanhamley]

Launch Checklist

• briefly describe the changes in this PR
  • Fixes Unused worker threads cause slowdown on machines with > 8 cores #7407
  • This came up recently when a user had a machine with 32 cores which spawned 16 workers, most of which weren't used but which degraded performance significantly
• write tests for all new functionality
  • Do we need tests for this? And how would we test it?

[ryanhamley]

What's our consensus on the max number of workers that should be spun up? @mollymerp suggested 6 and @mourner suggested 4. I assume the fewer is better as long as there isn't a limit below which we aren't spinning up enough workers. After gathering some customer feedback, I think 6 is probably an appropriate max number.

[mourner]

6 sounds good to me too.