In Roulette, selecting 12 numbers to bet on is almost one third of the numbers on a European wheel since, aside from the one green Zero, there are 36 numbered pockets, 1-36, organised on the betting layout as 12 numbers in each of the three Columns and 12 numbers in each of the three Dozens.
With this roulette betting strategy, you're going to be placing bets on the dozens of numbers, i.e. the first dozen (1 12), the second dozen (13 24), or the third dozen (14 36). If the ball lands on any number within the dozen you bet on, you win.