How many days must one fast to make up for the days one missed in the first Ramadan after puberty, given that the exact number is not remembered, and the ruling on breaking the fast at that time was unknown?

There is no need to make up for fasts missed before puberty. As for after puberty, if there is doubt about the validity of the fast after completing it, then the fast is valid and does not need to be made up; because the default assumption is that the fast is valid until there is certainty of its invalidity. However, if there is doubt about not having fasted certain days, then three days must be made up; because the default assumption is that the fast remains an obligation, and the obligation is not discharged except with certainty.