Even if the casino is using multiple decks, keeping track of the cards is a only a matter of counting. When the deck is low in fives, the player has a higher advantage than if it's sparse in any other card.
Logic dictates that, for a very simple card counting strategy, simply keep track of the fives.
Of course, there are several different card counting systems. Typically, the more complex a system is, the better your advantage can be, should you master it. However, the difference between two systems will usually be so small that ease of using the system becomes more important than gaining a tiny advantage. When a deck has a shortage of cards valued nine, ten, and Ace, the player is at a disadvantage. Bet more money when the deck is favorable to you.