If the deck is non-randomized, each play of the deck will take them through the whole deck.
If the deck is randomized, they will get a random selection of 20 cards each play. This random selection will pick cards the student has seen less frequently over cards they've seen more frequently so the student will run through the entire deck before seeing repeats. This means the first few run throughs of a deck with many cards will be harder than later run throughs. Teach your students not to give up!