Collect ‘Em All
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The Collect ‘Em All quest spans the entire game and is only completed once you collect every available card. You only need to collect each single card, meaning you don’t need to obtain all the duplicates to complete the quest.
There are 120 unique cards to collect with 79 duplicates (199 cards in total). You only need to find the 120 unique cards.
Gwent cards can be found by purchasing them from Merchants and winning games of Gwent against NPCs. After the Witcher 3 patch 1.20 the book A Miraculous Guide to Gwent can be used to track the number of cards still missing from your deck. Unfortunately it only gives you the broad area for the cards and not the exact location.