It's no secret that Fortnite is a monster, but because it's not on Steam it's impossible to say exactly how monstrous it really is, as Epic generally doesn't share those numbers. Epic Games Korea CEO Sung Chul Park recently did, however, telling Korean site Inven that the game has exceeded 8.3 million concurrent players.
The report doesn't get into specifics like when the peak was hit or how long it held, but it's such a ridiculous figure that it almost doesn't matter—especially considering that Fortnite only ("only") hit 3.4 million concurrents in February, when it surpassed PUBG's then-huge peak concurrent player record of 3.2 million, set in January.
And consider this: SteamDB indicates that there are currently 13.2 million concurrent users across all of Steam (though it peaks much higher at times), meaning that if Fortnite's growth continues along the same curve, there may soon be times when there are more people playing it simultaneously than there are on Steam—the whole damn platform. That is bizzonkers.
The original interview is in Korean and Google Translate is an ill-tempered beast, but an Epic rep confirmed that the figure is accurate.