klaus dice: As a paying subscriber number of cards you keep on the phone is not limited. It may be that you are rferring to the minimum number of active cards you want to have on the phone. That is a value you can set and which will add cards to your account as soon as there are less than the desired cards when you "Update cards".
The minimum number of active cards is variable between 0 and 100. If you would like to have more active cards on the phone you can simply add more flashcards manually (selecting them form existing word lists, or form the provided dictionary).