MAK or Multiple Activation Key allows multiple copies of Windows to get activated. However, they do have a limit. If you are receiving Error 0xC004C021 with the message:

The activation server reported that the Multiple Activation Key extension limit has been exceeded.

then it means that no more copies of Windows can be activated using this key. You will have to call your administrator or Microsoft Activation Cell Center to resolve this.

That said, chances are you will need a new key to activate your copy of Windows as MAK keys cannot be reused.