What is the ruling on someone who used to masturbate during the day in Ramadan, being ignorant of its prohibition and not knowing the number of days they did so, and are they required to fast or feed the poor?

We have previously clarified the impermissibility of masturbation, and that it invalidates the fast if the fasting person intentionally engages in it, knowing its prohibition. What you did out of ignorance of its ruling does not necessitate making up the fast or offering expiation.

However, for what you did after becoming aware of the ruling, you must make up the days on which you invalidated your fast by masturbation accompanied by the emission of semen. If you do not know the number of days, estimate them and fast what you believe is sufficient to clear your conscience. If you delayed making up the fast until the next Ramadan without a valid excuse, then you owe the minor expiation, which is feeding one poor person for each day.

If you doubt whether the masturbation occurred at a time when you knew its ruling, the basic principle is that nothing is required of you, because the default assumption is ignorance and lack of knowledge.