Is it obligatory to make up the missed prayers that the student abandoned during her early puberty, given that she does not remember their number, or is sincere repentance sufficient?

Praise be to Allah for guiding you to be diligent in prayer. Whatever prayers you missed after reaching puberty are considered a debt upon you that you must make up. If you are unable to determine what is incumbent upon you, then strive to ascertain and make up what will give you certainty or a strong presumption that you have fulfilled your obligation.