That would depend on your CPU. If you have a dual core, two threads are fine. If you should happen to have an i7 quadcore with hyper threading, eight threads would be fastest. If you tell me what you have, I will tell you how much threads are right.

A ship in port is safe; but that is not what ships are built for.
Sail out to sea and do new things.
-"Amazing Grace" Hopper

Avatar image of me done by Chidori.