Why twelve strikes
A game has ten frames. A strike in the tenth frame earns two fill balls, so a perfect game takes nine strikes in frames one to nine and three in the tenth: twelve in all.
Why 300
A strike scores 10 plus your next two balls. When those are strikes too, every frame is worth 30. Ten frames of 30 make 300, the highest score in tenpin bowling.
So close: 299 and 279
Eleven strikes and nine pins on the very last ball give 299. Nine pins on the first ball of the tenth, then a spare and a strike, give 279: that one ball also lowers frames eight and nine.
A 900 series
Three perfect games in one three-game series make 900, the highest series possible. It is one of the rarest feats in the sport.