That setting with a negative number lets you specify that Corona should not use a certain number of threads on your CPU, in this case let's say you have 32 threads, you are saying "Corona, you have to leave 4 threads free for Max and Windows and everything else on the system", so Corona would only use 28 threads.
You can put in a positive number too, e.g. if the above was true with 32 threads, entering 28 into that parameter does the same thing, saying "Corona, you can only have 28 threads, anything else is left for Max, Windows, and everything else" (in this case, leaving them 4 threads).
Put 0 in there, and you are saying "Corona, you can have all the threads."
It only applies during IR. There is another parameter that follows the same approach for what happens during final rendering.
PS the default is -1 for IR and usually works fine (in fact I often set mine to 0 and the system remains responsive). The default for final rendering is 0, Corona uses everything then.
