All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
According to the view of the majority of the scholars, you must make up all the days you did not fast last Ramadan and which you did not make up; this must be done before the beginning of the following Ramadan.
If you do not know for certain the number of days that you have to make up, then you should estimate the number and make up enough days with which you become certain or predominantly think that you are acquitted from liability. This is the only thing that you can do, and Allah does not burden a soul more than it can bear.
Hence, if you predominantly think that you did not break the fast in the previous years except the days that you mentioned, and that last year you only broke the fast for one single day, then you should act according to this, and you will be acquitted from sin by doing so; Allah willing.
Moreover, if you knew that it is forbidden to delay making up for the missed days of Ramadan until the advent of the following Ramadan without a sound reason, then you have to repent to Allah for delaying making up for the missed days.
According to the view of the majority of the scholars, you also have to feed a poor person for each day that you delayed making up, a Mudd of food, which is approximately 750 grams.
However, if you were unaware of the prohibition of delaying making up for the missed days [until the next Ramadan came], then you are not required to do anything, but you have to hasten in making up for whatever days that you had missed, as we have already mentioned.
For more benefit, please refer to Fataawa 114245, 101245, 23630, 136499 and 90795.
Allah knows best.