What is the ruling on the days that must be made up from Ramadan: are they the last six days or all twenty-six days? And is expiation obligatory if not all the days are made up before the next Ramadan?

Masturbation is forbidden, and repentance from it is obligatory. If you were ignorant of the prohibition of masturbation, then your fasting is valid and you are not required to make up the days. However, if you knew of its prohibition, then you must make up the days in which you masturbated before the next Ramadan. If you knew the ruling and delayed making up the days until the next Ramadan began, then in addition to making up the days, you must pay a fidyah (expiation) of feeding a poor person for each day. If you are unable to pay the fidyah, it remains a debt upon you until you are able to do so.