Krampus ate my d20s
This one is a kind of useless conjecture. The walls between BECMI, AD&D, and 2e were more of a curb than a wall. I imagine many more played D&D of some kind, if only for a time or two. Ryan Dancey's estimate range of 8-11 million seems too high but 4 million seems to be missing some data points to be clearer, namely Basic players.