There is no definitive way of knowing which one has more active players. Even if sub numbers were public and separated we wouldn’t know how many people actually play. I know a friend who still has an active sub and hasn’t logged in since 8.1.
But, after playing both I’d put my money on Classic WoW having more players. Seems more busy and lively to me, but can that can be based on so many factors as well. We will never know, but if I had to guess it would be Classic.
Of course there is - Blizzard simply isn’t sharing that information. We do know that subscriptions to wow (BFA+Classic) doubled around the time Classic was launched.
Umm…yes, that’s what I meant, sorry if I didn’t make that clear. They aren’t sharing the metrics, so WE can’t definitively prove it.
They’d have to share concurrent players too, not just sub numbers.
Subs =/= active players, which I stated, at least in my experience. I know someone , IRL friend, who hasn’t logged into retail in a very long time (I think right around 8.1) still has an active sub. I wouldn’t really consider that a player.
Right. They know all of this and, I suspect, won’t release it because the numbers no longer look good.
Well they stopped sharing them a while ago because of how hard they dropped and the backlash they received, so just imagine if Classic indeed had more concurrent players than retail, how much crap they’d get. 
It wouldn’t be good. Of course, I’m sure conversations about that very thing happened internally - and even if classic wasn’t more, if it was even 80% of the BFA concurrent players, that’s bad (for BFA and whoever designed it).
My tinfoil hat theory is Classic is more popular than retail, and that’s why they are “going” back to some of the philosophies classic had, without obviously changing the entire game to be Classic.
Level squish: a lot of people like smaller numbers and achievable level cap
Class Identity, not Spec Identity: returning certain abilities and unpruning
Making gear just gear: No titanforging, no corruptions, no azerite, just gear
Professions Importance: Said they are making crafting professions much more important
Those are just a few to name, but it seems to me like the success of Classic has definitely sparked some important conversations in the Shadowlands development conference room.
Oh, I don’t doubt it. We’ll have to see if they go far enough - or, are able to resist the urge to smear bonus systems all over everything.
Retail because Classic is stagnant and won’t age well as it gets worse.
The latest conference call indicated that retail is strong in the west (NA/EU), and classic is doing well in the east assuming the mmo champion summary can be believed. Don’t think I’ve seen anything to suggest either version of the game is landsliding the other.
Classic is more for casuals since everything about it is super easy aside from how long it takes to level. You know your game is a joke when leveling is the hardest part, I guarantee you BfA has more people
Neither. Probably a large amount of people who swap between both when they get bored of the other.
Surprisingly not everyone who plays retail hates Classic and vice versa.
Classic is quite active, but I think its lease on life might be limited with its current popular meta, which is to minmax to absurd extremes by collecting as many world buffs as possible and raidlogging. That playstyle reads as incredibly unsustainable — sooner than later people will start burning out, because at that point why are you even raiding? To collect world buffs ever more efficiently to raid more? There’s no point to having raid gear if you never actually do anything with it.
You said classic is hard. 
Wowprogress also tracks population numbers. It bases it on any guild that has any character that has killed at least 1 heroic boss (probably most active guilds tbh). Compare that to Ironforge Pro’s numbers and they look about equal. That’s not even mentioning the people in classic that don’t raid.
I’m not saying classic is better, I prefer retail over classic, but it is certainly very popular.
Hard to say… I know during buff times in classic stormwind and Org would be rocking with players…
Bosses have 3 mechanics and nothing has depth. Classic is a boring and worse version of WoW.
Nothing is hard in Classic. It just takes longer.
whispers
I sometimes wear that hat too.
Which is a lot. Once that’s done, though, might as well do something else.
I’m sorry, I have to laugh at this as it is the most ridiculous thing I have seen in a while.
That aside, I would bet money there are more players in retail than classic but it should not be viewed as competition. Anytime gamers have choices, we all win.