What is card counting? Do you mean counting 52 cards, and which is still on there? If not, then please express if possible. But if it is considered which card is left, then it is all ost impossible because they do not use single-time cards. There are multiple types of cards, some of them 3/4/5, no one knows which cards and how many. So, counting is impossible, I think.
It will be difficult for me to explain to you the method of counting cards in one message. It will be easier for you to use Google search or read about card counting here -
https://en.wikipedia.org/wiki/Card_counting But I recommend you to watch the movie "21", based on real events, starring Kevin Spacey and Jim Sturgess, which tells how students, led by their teacher, using the method of card counting, beat the casino.