Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Some scholars say that if you miss some days of Ramadan and another Ramadan comes before making up for those missed days from the previous Ramadan, then you have to make up for them along with a ransom.
The ransom is to feed a poor person one mudd from the predominant foodstuff of the country for each day. The mudd is approximately a little more than half a kilogram. This is the madh-hab (school) of the Shafi`is and the Hanbalis who act upon what has come from some of the Companions. The other Imams and scholars do not make this an obligation.
All in all, if this happens with a man, then he is to definitely make up for the fast days. As for feeding someone or giving a ransom, if he does this then he has done something good and if he abandons it then by the will of Allah there will be no blame on him. That is because there is nothing authentic from the Prophet (peace and blessings be upon him) concerning this.
As for having doubt about the number of missed fast days, the person should do what he thinks best and has certainty about. And so that he can be at ease concerning the preservation of his religion and have a clear conscience, he should fast a number that is more – and for this he will have a reward and recompense.
Almighty Allah knows best.