Is it necessary to make up for the fasting days that followed masturbation without ghusl, given the uncertainty of semen emission and the unknown number of days?

Masturbation is forbidden, and one must repent from it. Ghusl (definition) (ritual bath) is only obligatory upon the of discharge of semen, and there is no ghusl required if there is doubt about its discharge. If you are not certain that semen was discharged, then neither ghusl nor making up prayers is required of you, and your fast is valid. Janabah (definition) (state of ritual impurity requiring ghusl) does not affect the fast; if a person wakes up in a state of janabah, their fast is valid, and they must perform ghusl for prayers.