The number is most likely from Steamspy
It lists the 2-5 million owners. Note, that number is steam only. I can't find anything on GoG, Xbox, PS4, or the Switch.
Yeah, I already confirmed where it was from. And yeah, that range is absurdly broad because SteamSpy is not as reliable as it used to be before most Steam profiles became private by default (since scanning those is what it uses to gather data).
While once it used to have a margin of accuracy of 5% that became more and more fine tuned for big sellers, now that's not case anymore.
Still, it can still offer some vaguely reliable data as a broad reference. In the specific case we know that at very least it sold two million copies and that they are probably even more (but it's pointless to attempt a guess without confirmations from Larian, which as a private company they have no obligation to offer).
Anyway, back to the point of what we can safely deduce from this: at very least, even taking the worst case scenario into consideration, one where we are imagining that sales topped at 2 million on Steam and amount to virtually nothing on other stores and platforms, we can say that DOS 2 can easily match and probably surpass sales of Dragon Age Origins on PC (which were not the majority). And possibly compete with its sales combined on all platforms and surpass them, too.
I think DA: O could have even reached 10 million, only problem is that 11 years have passed since the premiere.
Over the years it was also given for free to lure people on Origin and sold for five bucks countless times. And I still wouldn't bet in a similar number or Bioware would scream that from the rooftop at full lungs every time they mention the franchise.