FWIW, I think it depends a lot on what you're using the two OSs for. If you'll keep Windows open a lot in the background and use OS X a lot simultaneously, then just give Windows 1 core. But there are lots of variations on this, you might use the Windows most of the time while it's open and then you can do the opposite.
So I can change this accordingly, right now I assigned 2 out of the available 4. Lets see how this plays out. I can always change this settings, no risk of deactivating my windows 7 due to hardware changes? what about the RAM? I have assigned 6GB out of 12GB, again, no problem if I change it sometimes?
They're shouldn't be any issues in changing the hardware allocations. Unless you change the size of your windows partition.
Unfortunately, quite false.
Specifically regarding the OP's question: Windows will detect if you change core count. You will need to re-activate Windows.
Based on my experience, you can change the allocated RAM, and the disk size, but not core count, and you may have activation issues if you change 3D acceleration setting.
I have a 2011 27"iMac with core i5 (3.1 Ghz), how many cores should I assign to my Boot Camp partition linked to Parallels 6 (Windows 7 Home Premium 64-bit)
Specifically regarding the OP's question: Windows will detect if you change core count. You will need to re-activate Windows.
I'm sorry, but that is absolutely false.
Through the various builds of Parallels 6, I've changed from 1 to 2 cores and back to 1, numerous times. I've also upgraded the memory, twice. Windows 7 has detected the changes, but not once have I had to re-activate Windows 7.