http://www.techradar.com/news/playerunknowns-battlegrounds-sets-new-steam-record-with-13-million-concurrents

The hugely successful battle royale multiplayer PlayerUnknown's Battlegrounds has taken the title of most concurrent players on Steam. It knocked Valve’s own game Dota 2 off the top spot with over 1.3 million players.

If you’re interested in specific number crunching, at the peak there were 1,342,857 people all vying for survival at exactly the same time.