بسم الله الرحمن الرحيم
If someone intended that if he commits sin he will keep hundred fasts, and then he committed the same sin, so though these fasts are not wajib (compulsory) on him, but it is better for him to keep the fasts one by one. However, if one is unable to do so, then it is better that he pays wheat equal to Sadaqah al-Fitr (1.633 kg) or its price for one fast to the poor and needy ones. Also, you should keep on doing true taubah from the sin you committed. If it is hard to fast in summer days, you may keep them in winter; however it will also suffice if you give fidyah (expiation).
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.