1.5 million people play on private servers. That is a good starting point.
Use Google Trends compare WoW Classic to WoW BFA. Plus Activision would not bother doing this for a few hundred thousand subs.
Its safe to assume based off empirical evidence that Classic will have 2 million at launch. But you say “That’s FIIIIIINE. We don’t need sharding. Because there wasn’t sharding back then.” Despite the fact that at no point in the history of the game did 1 million people all start at the same level in the same few zones… let alone a reasonable 2 million.