Must I make up the fasts from the days when I bled before reaching puberty, and how many days must be made up if necessary?

The basic principle is that the blood you saw is menstrual blood. You should have refrained from prayer and fasting, and then performed ghusl after it stopped. Since you did not do so, you are obligated to make up for the fasts of the days you saw the blood. As for the prayers you performed on the days the blood stopped before performing ghusl, there is a difference of opinion among scholars regarding making them up. If you do not know the number of days you saw blood in Ramadan, then you should make an earnest estimation and make up what you are certain of, or what you predominantly believe, will clear your conscience.