How many threads are optimal? That depends a lot on the program - what it does, and how many processors are available. There is no way to really predict it. The best thing you can do is make your program with a configurable number of Threads (a
Thread Pool comes to mind - see ExecutorService for more). Then
test the number of Threads that get you the correct performance.
You can also get a bit more detailed, depending on the work that you are doing. You may need to balance threads that 'get' data (producers) and threads that use data (consumers) - or threads that use I/O like disks, or databases, with those that don't. So there is no magic number, you have to code and test to see.