Is a deck with extra aces more beneficial to a dealer, or a player?
This is a very complicated question. There are several factors to consider that make it impossible to answer without knowing a lot of details that you left out. Taken to the extreme, imagine a blackjack deck with 1 million extra Aces. In such a game, the player would have the advantage if the dealer was required to hold on 17. The player can hit until they have 18, confident that the dealer would most likely get 17 and be forced to stop. Here are some odds for adding a single extra Ace.


Decreased Chance of Player Blackjack


52 card: ((4/52) × (16/51)) + ((16/52) × (4/51)) = 4.82655%
53 card: ((5/53) × (16/52)) + ((16/53) × (5/52)) = 5.80552%



The odds of getting BJ are the odds of getting dealt an Ace then a 10/face-card plus the reverse order. Adding in an extra Ace increases your odds by almost one percent.


Increased Chance of Dealer Blackjack when player has BJ


52 card: ((3/50) × (15/49)) + ((15/50) × (3/49)) = 3.67347%
53 card: ((4/51) × (15/50)) + ((15/51) × (4/50)) = 4.70588%



The calculation here is much the same, except two cards are removed (the player BJ). Adding a single Ace hurts the player by over a whole percentage point.

Unless you give me a lot more detail (there are other things to consider) I have no idea who has the advantage. For example with 4 Aces, you have increased your odds of BJ by almost 3.5%, but the dealer has an increase of greater than 3.5% chance of having BJ when you have BJ. Who has the advantage would require extra information on what the payout is for player BJ.


56 card player BJ: ((8/56) × (16/55)) + ((16/56) × (8/55)) = 8.31169%
56 card dealer BJ after player: ((7/54) × (15/53)) + ((15/54) × (7/53)) = 7.33753%