Is it obligatory to make up for past Ramadan days due to doubt about having masturbated during them, and how is the number of days determined if making them up is obligatory?

Do not pay attention to whispers [waswasah]. If someone does something that breaks their fast out of ignorance, they do not have to make up the fast, and their fast is valid. Doubt about an act of worship after completing it does not affect its validity; the default is that the fast is sound.