Many decks were used by casinos in an effort to foil card counters. Well done, Casinos! We only need to convert our data into a “True Count” or count per deck in order to use our running count in a multiple deck game. Below is a breakdown of each stage.
Something is very different when you have a +5 running count with 6 decks left from when you have a +5 running count and only 1 deck left. The player does not yet have the edge in the first case because there is less than one more high card in each deck. A card counter is concerned with the concentration of high cards relative to the concentration of low cards, not just the quantity of extra high cards.
We therefore know that there are 5 more 10s and Aces in the 52 cards that remain if you have a running count of 5 with only 1 deck left, giving the player the advantage! This is the reason the “True Count” is so important.