Do you know if your system has hyperthreading on? That will double the cores it thinks are there.
An easy way to find out how many to use is just fire up taskmgr and look at how many processors it displays in the performance tab.
For example I have a laptop with a i7 in it with 4 cores hyperthreaded so it shows 8. Which is what I would use if the target I am analyzing is that processor.