What should one who vowed to fast three days for every time he committed a specific sin do, then committed it many times and does not remember the number?

This vow is of the nature of a vow of obstinacy and anger. If your friend breaks it, he must fulfill his vow by fasting for three days, or by performing an expiation for breaking an oath (kaffarat yameen). The expiation for breaking an oath is to feed ten poor persons, or to clothe them, or to free a slave. If one cannot find these, then one must fast for three days. Since he obligated himself to fast for three days for each instance of this sin, he is bound by what results from this vow every time he commits what he vowed not to do. If he does not know the number of vows he has broken, he must investigate and strive to estimate them. If he cannot determine them precisely, he is only obligated by what he is certain of. He must also be wary of sins and keep away from them.