Adverse to traditional belief, card counting in Blackjack is not illegal. Another very different thing is that it is prohibited in some physical casinos or that it is frowned upon.
Of course, you need a very good memory and a great ability to process information quickly. This is obtained with a lot of exercises and with mnemonic exercises. It will be more interesting to play blackjack online.
However, keep in mind that some blackjack games use multiple decks, which would make counting very difficult.
Here is a brief explanation of how to count cards :
The most famous method of counting cards is the High-Low card counting system. This consists of giving a numerical value to each card so that they are divided into 3 groups:
From 2 to 9 = +1
From 7 to 9 = 0
From 10 to A = -1
If we add up the entire deck taking these values into account, the result will be zero. Thus, with this method of counting cards for Black Jack, it is possible to know, in a general way, what type of cards are left in the deck: if they are low or high cards.
When the croupier has already removed half of the deck, if the resulting value of the count is high, it implies that there are more high cards (10, face cards and aces) than low cards.
This gives important information to the player, as he will be able to review his strategy taking into account this information: when there are many high cards left in the deck, it is an advantage for the player against the dealer since he has a greater chance of getting a 21 blackjack.
