What is the difference between "cards per set" and "quantity" on the flashcard ordering page?

Cards per set is the total number of cards you would like in a single set of flashcards

Quantity refers to the total number of sets you would like to order.

So if you were looking to order flashcards featuring 26 cards in a set and wanted 16 complete sets you would select cards per set: 26 and quantity: 16.