How can one make up for days of fasting missed in Ramadan due to masturbation, given a current medical exemption from fasting and an inability to recall the exact number of days?

Masturbation is forbidden and invalidates the fast if it leads to the emission of semen.

If you engaged in it out of ignorance of its prohibition, you are not required to make up the fasts. However, after becoming aware of its ruling, you are required to make up the fasts if semen was emitted.

If you do not know the number of days you need to make up, then make up what will ensure certainty or a strong presumption of having cleared your obligation.

You are not required to pay the expiation for delaying the make-up fasts as long as you are unable to do so.