I have missed fasts in Ramadan deliberately, a couple of years ago, but cannot remember exactly how many though they were at least 12 days. What should I do?
Bismihi ta'ala
If what you mean by deliberately is that you did not have a valid excuse, then you must:
1. Repent to Allah ta'ala, asking for His forgiveness, if you have not done it yet.
2. Pay the kaffarah, of feeding of 60 poor people.
3. Redo those days, as qadha', and you redo only the days you are certain of, which would be the least amount. In this question, it would be 12 days of qadha`.
And Allah knows best.