:disagree: No difference here. For each computer, a separate license is needed.
This is different to some MS Office products: MS Office Home&Student can be installed on up to three computers (under certain conditions I am not aware of).
If you are concerned that you will need to install on several machines-then get a multi-user pack of the OS ( more money but completely legal for the number of machines).
:iagree: System Builder versions are often sold as 3 or 5 pack. (seen them at Amazon)