Note that the GP wrote "that would buy the game" which I think makes this unknowable / unmeasurable. You can measure how many players have pirated the game but not how many of them wouln't have bothered at all if they had had to pay for it.

Take a person that has pirated many games, how do you know which ones they would have actually paid for? Even if you knew the ones they play the most, it woulnd't necessarily translate to a buy in the same order.