Depends how exact you want to be
2.7 cards per hand is a good rule of thumb.
The dealer uses more cards than the players. Basic Strategy says to stand on 12-16 against some dealer up cards, while the dealer will always be hitting these hands, using more cards. A table with 5 players and the dealer (6 hands total) doesn't use exactly twice as many cards as a table with 2 players and the dealer (3 hands total).
Also a player playing heads up with the dealer, the table will use less cards on average, because the dealer doesn't play out his hand when the player gets blackjack or busts.
If the dealer peeks under the 10 for blackjack, then less cards are used, because players do not play out there hands when the dealer has blackjack with a ten up.
If the dealer hits soft 17, both the player and dealer will use more cards on average.
If the games is DOA or allows surrender fewer cards are used by the player.
DAS and RSA, allows the player to use more cards.