By lets you decide how this restriction is applied. “Maximum unlearned” lets you select a total number of non-“learned” cards to be active at once; once you’ve reviewed a card, even once, it’ll be part of your pool of cards to study, so if you’re already working on more than the specified number of “unlearned” cards before you enable this option, it won’t take any away, but it won’t introduce any new ones until you’ve learned enough of them to drop the un-“learned” count below the level you set. “New per day” lets you set a number of new cards to be introduced per day; it only does this on days when you study, so if you don’t review flashcards for a few days you’ll only find one day’s worth of new cards waiting for you when you pick up studying again.