It is mandatory to make qaza of all the fasts which you have missed after you turned mature and all the prayer of the days when you were paak. You should do taubah and istighfar and offer all the missed salah and sawm as soon as possible. It is the kaffarah. If you do not remember the exact number then you should calculate the maximum prayers and sawm you might have missed. You should offer that much salah and sawm about which you are sure. If the number is more and it is impossible to make them in one go then you should offer one qaza salah with the present salah and keep the fasts when the days are shorter during winter. If you are healthy enough to keep the fast and offer salah then it shall not be sufficient to give the fidyah.
Allah knows Best!
Darul Uloom Deoband