The most players online simultaneously on one MOG server were 190,541 players on the RU2 server of World of Tanks by Wargaming.net LLP (UK) on 21 January 2013.
The number of players was reached at 19:31:02 (Minsk time) (16:31:02 UTC). RU2 is part of the Russian cluster of servers for the game. It is physically located in Moscow, Russia.
Comments below may relate to previous holders of this record.