Shannon reposted his response
Ben starts a great discussion on D&D players then & now.
Here was my own response, which of course is highly speculative too:
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I think it's possible the classic D&D players could be an order of magnitude higher.
To start with, 4 wasn't the standard group size then. It maybe averaged 6-8 based on the player numbers on modules, but anecdotally people often talked about 10-20 member groups.
And then you have to start looking at not just parallel players, but sequential players (e.g., how many players did a single GM lead over time).
That's of course lessened by the number of players who also bought DMGs, but that was also lower, particularly in the '80s. But I think 4 million could easily become 10 million or 20 million or even 40 million.
(But a lovely discussion point whatever the answer is; thank you!)
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