Playing multiple hands in Blackjack can increase the house edge because it exposes you to more risk, as each additional hand is an independent bet. It can also diminish the effectiveness of card counting, especially in games with fewer decks. Additionally, managing strategy across multiple hands can be more complex, leading to deviations from basic strategy, which might lower your expected return. Lastly, betting on more hands raises your overall stake, potentially amplifying losses, which further contributes to the increased house edge.