6 billion sounds kind of arbitrary, but yes, hundreds of millions actually. It really just depends on how long one is willing to drive. Also, judging by site traffic, the odds are pretty good you would be in the United States. Not to mention, even if you were outside of the United States, I’m not exactly sure why you would be coming to a site headquartered in the United States, frequented mostly by people in the United States, asking where you could physically go to buy a book, outside the United States. (And not even mention this important detail.)
So I’m not quite sure where this “6 billion” number comes from. But yes, judging by the evidence I mentioned above, one would assume you were looking for a place to purchase the book in the United States, and so yes, it’s entirely possible that you were one of a few tens of millions out of maybe 310 million.
I don’t see why that is so far fetched. (Not to mention, I was simply answering the questions you asked.)