Obvious there is no official data that we can go on, but from what I see, only hardcore fans buy multiple versions. There is no way there is only 7-8 million Pokemon fans
Yeah definitely most people don't buy both games, only the really hardcore fans do that. That's what trading is for. And I'm sure for those people who do buy both games many of them would be smart about it and just got buy a used copy after they've beaten the one they bought. Rather than the player base being half of what the sales suggest as BoseDK says it's probably like 90%+.