What should one who vowed to fast for two or three days, and whose wish was granted, and who fasted what he vowed, do if he forgot the number of days he fasted?

Doubt about the number of days a person fasted after completing their vow does not have an effect. This is because doubt concerning an act of worship after its completion does not matter. This is due to the fact that the apparent understanding of the actions of those bound by religious obligations regarding acts of worship is that they are performed perfectly. Furthermore, differentiating between doubt during an act and doubt after an act does not necessitate repeating the latter, as that would lead to hardship.