In roulette, there are 36 numbers (0-35), which are spread across 18 black and 18 red numbers. Roulette players can wager on any number from 0-19, 21, or even 25.
Although there is no one “right” answer to this question, it is generally accepted that bettors should place smaller wagers on the low numbers (0-9) and larger bets on the high numbers (10-19). Additionally, it is advised to avoid placing bets on any number that appears twice consecutively.
