Is doubting the number of rak'ahs and building upon the lesser amount incorrect, and does it necessitate re-performing the prayers, whether by building upon the correctness of the preceding part of the prayer or by acting according to the first thought that comes to mind?

The majority of jurists hold the view that whoever doubts the number of rak'ahs (units of prayer) should build upon the lesser number and perform the two prostrations of forgetfulness ( sujud al-sahw) before the taslim (salutation), based on the hadith of Abu Sa'id. However, this applies to someone who experiences doubt occasionally. As for someone who is afflicted by doubt daily, they should disregard it and build upon the greater number. Even if they build upon the lesser number, their prayer is not invalidated, because they have returned to the original principle.