The number of decks matters a lot. You can see from the table above on rule variations that the house edge is reduced by 0.19% going from six decks to two and 0.48% going from six decks to one. That is a lot, especially compared to the thin house advantage in blackjack to begin with. One reason is that the probability of a blackjack goes up as the number of decks goes down. This is due to the effect of removal. While the player will get a winning blackjack just as often as the dealer does, the player wins 3-2 when he gets one and loses 1-1 only when the dealer does. The value of getting the extra half unit is worth half the probability of a player winning blackjack. This is 4.65% in single deck blackjack and 4.53% with six decks, for a value of (4.65% - 4.53%)/2 = 0.06%. Another reason comes from more effective doubles with fewer decks. If you are doubling, you clearly don’t have any tens in hour hand and the dealer probably isn’t showing one either. That makes the rest of the deck richer in tens, which increases your chances of getting a ten after doubling. That effect of removal is more powerful the fewer the decks. I roughly estimate the benefit of that to be 0.08%. That still leaves 0.34% unaccounted for. I think that the remaining difference again gets back to the stronger effect of removal as the number of decks decreases. If both the player and dealer get stiff starting hands (12 to 16 points), then if the player hits and gets a good card(s), then he leaves less good cards for the dealer in the remainder of the deck. For example, here is the expected value of a player 10-6 against a dealer 6 by number of decks:
1 deck: -50.69%
8 decks: -53.60%
Note the expected value is 2.91% higher in a single-deck game. Such situations occur often. I plan to look deeper into this in the near future.