The number of decks in Blackjack affects the odds by influencing the probability of drawing certain cards and impacting the house edge. In games with fewer decks, players have a better chance of getting a Blackjack, as there are more high cards (Aces and 10-value cards) in proportion to the deck size. This increases the player's odds and lowers the house edge, which can be as low as 0.17% in single-deck games. Additionally, fewer decks make it easier for card counters to track the remaining cards, enhancing their advantage. In contrast, multi-deck games reduce the effectiveness of card counting and slightly increase the house edge, typically around 0.5%. Therefore, fewer decks generally benefit the player, though the rules of the specific game can also play a significant role.