I would say default to min(affinity, system threads, 16) and let the user input a specific number to override it. I had a problem recently where a library star

I would say default to min(affinity, system threads, 16) and let the user input a specific number to override it. I had a problem recently where a library started a process per CPU thread I suspect the author didn't intend it to start start 256... Affinity is only easy to get on some platforms but you should use it instead of the system threads if available.