You can't do that however, as one classic realm could be equal to 5-10 retail high pop realms.
Blizzard changed the way u see population after classic popularity at launch, i can link u source.
In retail servers show populations in relation to each other.
That means retail could have 10k players and still have more high pop realms than classic
Wowhead have a article posted about this 1 week after classic launch.
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I would say its highely unlikely retail have more players than classic. If you look at twitch as thats the best metric we have atm, you can see classic out performs retail 10:1.
Remember how big classic realms are, and even now 2 months after launch we still have layering on over half of the servers.
If you compare this to bfa launch, people started unsubbing day 5, after they completed all content.