The number of cards you need to turn in for the reward is visible on the card itself. If you have a full stack of cards in your inventory or stash, the value on the icon will be blue instead of the usual white. Once you have enough, Tasuni in Highgate will accept them and reward you with the prize on the card.
Divination cards are items found throughout Path of Exile that can be turned in for a reward. Different divination cards reward you with different things, including currency, specific item basetypes, other divination cards, and even unique items that cant normally be found. There are even some items that can only be obtained by collecting the right cards. Every divination card was designed with the help of our supporters, though the rarity and drop locations were determined by Path of Exile developers. Some divination cards can only be found in certain locations or from certain enemies, so pay attention to where you are when you find them. If theres one you really like, you might consider visiting that area frequently to find more.