What are the rulings regarding expiation for delaying the making up of missed Ramadan fasts, and must I re-fast days whose number I do not remember?

Only the days a fasting person missed need to be made up. If the next Ramadan (definition) arrives, there is no obligation to re-make up what was already made up. Rather, according to the majority of scholars, an expiation (kaffarah (definition)) of feeding one poor person for each day whose make-up was delayed without a valid excuse until the next Ramadan began becomes obligatory. The expiation does not, on the soundest view, repeat with the recurrence of Ramadans. If she was ignorant of the impermissibility of delaying the make-up, then there is no expiation upon her.