There is one simple rule: You need one registration key for each computer that is monitored at the same time.
Registration keys for Hardware Monitor and Hardware Monitor Remote are interchangeable. So if you are running Hardware Monitor and Hardware Monitor Remote on one computer you’ll need only one key for this system. If another copy of Hardware Monitor connects to this system, it will need a second key because then we’ll have a network of two monitored computers which are in use at the same time.
Another example: You have 2 computers that should be capable of monitoring each other. In this case you install the applications 4 times (a copy of Hardware Monitor and a copy of Hardware Monitor Remote on each of the systems), but you only need to buy 2 registration keys.
Further information: Monitoring Remote Computers