What is the ruling regarding the remaining days of fasting to be made up, the number of which is not remembered due to not having made up the fast of Ramadan days in some past years?

If the delay in making up the fast is without a valid excuse, you are required to pay a fidyah (expiation) for each day of delay until the next Ramadan, regardless of whether you have already made up the missed fasts or not. However, if the inability to make up the fast was due to a valid excuse, such as illness or ignorance, then no fidyah is required of you.