It’s not easy to calculate exactly ones left fasts but it depends on his/her past lifestyle, I mean if you are sure that the environment you lived for example 20 years and you are sure that in those years you had not fasted then you will have somehow an exact number but if you know that some time you had fast and some times you had not fasting, then calculate based on that.
Shariya has given relaxation here you may choose least numbers in those years you left your fasts.
As for as Kaffārah concern it is 750 grams wheat for each fasts if you are not capable say Qadha for your lest fasts.
This Kaffārah food should be given to Miskin from Imamia otherwise if not possible you can give to at least a practicing Muslim.