Is atonement or fulfillment obligatory for one who vowed to memorize 100 surahs from the Qur’an, then realized that the number of surahs in the Qur’an is less than he had thought at the time of the vow?

Your vow is a vow of obedience and righteousness, and it must be fulfilled if you had reached the age of maturity when you made the vow. Your ignorance of the number of chapters (surahs) in the Quran does not affect it. However, if the vow was made by you during childhood, then it is not valid and not binding, unless you were discerning and understood the concept of drawing closer to Allah, in which case it is recommended for you to fulfill it. There is no expiation for this vow unless you are completely unable to fulfill it, in which case you owe the expiation for breaking an oath. We advise you to be diligent in memorizing the Holy Quran.