In 2021 DDB allegedly had close to ten million "users," which seems like an awful lot; I am kind of skeptical. However, the paid subscriber base being 5-10% of that does not seem unreasonable. My paid subscription to DDB props up an awful lot of free accounts. In 2023, Chris Cocks claimed that, by Hasbro's reckoning, close to 50% of D&D games now use some form of digital tabletop, mostly DDB.
Edit: I suspect a lot of those DDB free accounts were seldom used and since abandoned. I would love to know how many regularly active accounts there are. It seems to me that the subscription numbers should be out there somewhere, as Hasbro is a publicly traded company.