If you want to decrease the quantity of a card in your collection, check out this article.

Remember to use the grouping options to ensure you are removing the desired number of cards.

Apps:

To remove a card from the staging area, swipe left on the appropriate card. A red bin icon will appear that you can tap to remove the card.

To delete a card that's in your collection, tap on the appropriate card. Tap the Edit button, scroll down and tap the red Remove button.

Web:

To remove a card from the staging area (on the Build page), click on the X icon on the right of the appropriate card in the appropriate Session. You can remove all the cards from a particular Session by clicking the "Delete" button for that Sesssion. You can also remove all the cards from every Session by clicking the "+ Clear All" button.

To remove a card that's in your collection, find the appropriate card and click on it to open the details panel. Click Edit and then Remove. You can remove multiple cards by selecting them and clicking View Selection, then Edit, and then Remove.