There is no stack of unused cards, just as there are no words you decide not to use in your conversations (well if you are very systematic, you might do that!
)
The never-known cards appear in grey hatched color so they are easy to localize, and the very well known cards remain in the rightmost column, with an increasing refresh interval (in ColorMode RepeatAdvisor).
Of course you can move the cards around as you wish. Stackz lets you do those things easily, it rather tries to visualize the status than automatically doing decisions for you.
The capacity of the stacks is only limited by your system memory, which is a lot. But generally it does not make sense to add more than "a mentally manageable" amount of cards to one lesson.