In the case of card counting, multiple decks are something of a boon and a bane to card counters. In the event that all other rules are equal, then the more decks a Blackjack game has, the greater the House Edge will be against the player, which is to say that there will be a lesser return to player.
As we have discussed before, the number of decks with which a Blackjack game is played is essentially a rule unto itself and impacts the House Edge of the game accordingly.
