What is required of one who vowed to fast a number of days but cannot remember how many?

Whoever vows obedience to Allah, it is obligatory for him to fulfill it.

If you vowed to fast a number of days but do not remember how many, then strive to ascertain them. If you are unable to determine them, then fast what you believe most likely discharges your obligation.

However, if the days were specific and you missed fasting them without a valid excuse, then you are obliged to make them up and offer an expiation for a broken oath.

If the omission was due to an excuse, then the preponderant opinion is that only making them up is required.