What is incumbent upon one who vowed to abandon a sin, but does not know the extent of what he vowed, and swore an unknown number of times, but presumes it to be 100, and did not fulfill his vow?

There are two issues related to vows (nadhr) and oaths (ayman):

First Issue: Vowing to abstain from sin Abstaining from sins is a religious obligation, and vowing to do so only confirms its obligation. If you vowed to abstain from a sin then committed it, you must seek forgiveness and repent. The majority of scholars believe that no expiation (kaffarah) is required of you, because a vow to abstain from what is forbidden is not valid, and it is not permissible to commit to something that is already an obligation. If you believe that expiation is required, then you must strive to ascertain the number of vows to offer expiation. If you are unsure of the number, take the minimum certain number, keeping in mind that repeating a vow concerning the same thing only requires one expiation.

Second Issue: Repeated oaths If the repeated oaths are concerning one matter, the predominant view is that only one expiation is required upon breaking the oath. However, if the oaths are concerning multiple matters, then expiation multiplies with the multiplicity of what was sworn upon or about, and the breaking of each oath. If you do not know the exact number of oaths, you must strive to determine them, and take the minimum certain number, because the default principle is the acquittal of responsibility.