Not to mention each deck is only used once. Then they usaly punch a hole through it so they know not to use it again, take it to the gift shop and seel it to tourists as a keep sake of thier trip.

To count trough 5 to 6 decks (not knowing exactly haw many is damn near impossible)

When playing with friends though

Okay there are 52 cards in a deck.

Any decent spades player knows 13 per suit.

Sooo all you have to do is chill a couple hands. Pay attention to cards and thier suits. You will not be 100% sure of the people who fold but if you play the game enough you will be able to closely guess what they had.

Keep a running list in your head

how many kings have come out?
how many queens
howmany jacks and 10's
how many black jacks

If you are shuffleing after every hand, pay attention to ever crease and nick in your cards. Rember the important cards and how the back looks. Hell bend a few(oops sorry guys I got carried away)

Play spade with partners often you will be able to count cards in your sleep. the whole game is based on counting cards and reading your team mate and the other players faces.
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