In what case does card counting on blackjack work?

I have read couple of information about card counting system. Then I have known that nowadays card counting system doesn’t work that well. So my question is in what case does it work? For example : How many decks of card is good for card counting for make it works? Also is it good if dealer has a shoe? What is a edge on blackjack? What is a dealt? Please give me information on those. Thank you.

Same answer as to the last guy. Google blackjack + cards + count and let someone smarter than me tell you.

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