Most machines use 5-8 decks. The cards are reshuffled after every round so it is possible to see the same cards right away. Some machines have a 10-20 card delay, but that doesn’t help a card counter at all. Unless the dealer plays at least half of the cards before putting them back in the machine you will be wasting your time trying to count the cards.
If you want more information about these machines, find out which one your casino uses and look up the patent information. That will give you a general idea of how it works.
-Sonny-