Only a million?
That seems awfully low for a company to claim 50 million players if they've only sold a million PHB's.
Excellent question. We don't actually know the number sold. Stan! says around 800k. I've heard over a million in 2017. I think 2017 is around the time 5E really took off. I would guess that shortly after this is when it stopped feeling at all like a regular game. But, it's just my guess.
I also suspect the 50 million is a number based on various factors and extrapolations, including anyone who ever played. They aren't necessarily customers. (Edit: and, as others have pointed out, it's likely a number of all players ever, not current active 5E players.)
Here's the kind of thing that can happen. During 3E I played in the Living Greyhawk organized play campaign. I moved to Portland, and there were about 80 hard-core D&D players in the area playing the campaign. This being my world-view, I assumed that was the entirety of people in the area interested in playing D&D in a public space.
So, 4E launches and we have the Encounters store program. I volunteered to help organize it at my FLGS, and we quickly have 4 and then 6 tables. I realize I know a single person. It was my job to report numbers to the Wizards Play Network, so I started looking at the numbers. In a single season we had more than 300 unique players. Of these, I knew 6 of them. That's how much tunnel-vision I had regarding what I thought of as possible players in my city. We went on to have many more players (though I was no longer tracking numbers). It's safe to say we have thousands in the area. They come, they go, they may or may not buy stuff. But the potential market is enormous.
So, 50 million may be a made up corporate number, or even a bad extrapolation. Or it may be accurate. Either way, it suggests to me the potential audience for D&D (and, to a lesser extent, any RPG) is enormous.