Hello,
MAK keys have a number of allowed activations available. The number of activation on each Product key is determined by the Volume Licensing agreement or via the program under which that product key was distributed.
Each time you activate using that product key, the number of allowed activations are decremented by one, regardless of the machine that it is activated on.
Not sure how you would determine how many activations you would have remaining on that product key without making a call to Microsoft.
Darrell