Must one make up for the votive fast after repenting from masturbating during puberty, given that one does not remember the number of times it occurred?

Masturbation is forbidden and harmful, and repentance from it is good. As for the vow, the one who makes a vow of "lajaj" (a vow made out of anger or compulsion) has the choice between fulfilling what they committed to (fasting) or performing an expiation for an oath (kaffarat yameen). You must perform an expiation for an oath now along with repentance, or fast what you vowed until the doubt is removed. If this act occurred before puberty, you are not held accountable.