To answer your question you need to determine which of the following two scenarios your setup falls under:

1) If you are running multiple instances of Windows on your computer, the operating systems may "recognize" the hardware in the same way. To determine whether your hardware is being recognized as the same on both OSs you will need to look under the hardware profiles (in Windows usually called "Device Manager") and compare each listing to one another to see if the hardware is listed in the same fashion. If the hardware profiles are the same you may be able to use the same registration number from one OS installation to the other. As hardware choices for PC based computers are so varied, this may not be applicable to every situation and you still may need to purchase more than one seat.

2) If you are running multiple instances of different OSs (Win 7 and Mac OS X 10.4 for example) you will actually need two different versions of Net Nanny. You will need one Net Nanny version for Windows and one Net Nanny version for Mac.