Someone on the classic forum made post that stated classic now has 4.5 million active players.
Thread is here - https://us.forums.blizzard.com/en/wow/t/4-5-million-and-rising-fast/298970
He explains where he got the info from in the thread. I hope the idiotic, "You think you do, but you don't" people learn to keep their idiot mouths shut.
Some other interesting info is that only 27k people are level 60 out of those 4.5 million captured.