Must be a vast number of Product Keys in circulation.
The maximum number available is easily worked out (with a calculator!)
Numbers represent 10 choices and letters 26. Starting with, for instance, a letter and then a number would be 26x10 and so on for the first block of five letters and numbers.
This carries on with the subsequent blocks, multiplying by 10 or 26 as appropriate. The calculator may not be able to handle the full range...:-)