Let's talk big.
Not just shooters (which would contribute a lot)
Superpower 2: Want to launch a nuke? OKAY! There goes a huge chunk of population in a short time.
Ace Combat Zero: In one of the earlier sequences, the enemy country nukes its own borders. Plus, if you lose the last mission, almost the entire world gets killed.
Civilization games: If one turn is a year, then there'd be a whole lot of death by natural causes (exceeded by the birth rate) by the end of a game.
Plus, every time a nuke-booster in Modern Warfare 2 succeeds (which is quite often, unfortunately), you have to think of how many people would die.
Plus, you never mentioned what kind of game it is. Imagine a bunch of psychopaths playing Dungeons and Dragons (almost redundant there). Someone could easily say that there are a septillion people in the world. Whoops, they just all died due to a meteor destroying the planet.
I'd dare say the numbers would be colossal, even if it were just video gaming. So yeah, biggest available number.