More than 20 millions deaths...the strangest part is that MC is still somehow sane, I guess the pure white art was he shutting off and just letting it happen.
The meaning of the number of deaths is clear, he somehow experienced one of two things:
1) every possible variation of the turn based battle the game world he is in is following
2) every defeat his character experienced in the game of every player that got to that point, it should match a game with a decent number of copies sold, and if it was something like Final fantasy it would probably be 10 times higher