question #1 Sahih al-Bukhari 1232, Book 22, Hadith 10 : What should a person do if they become doubtful about the number of rak’ahs they have prayed during Salah?

Allah’s Messenger (ﷺ) said, “When anyone of you stands for the prayers, Satan comes and puts him in doubts
till he forgets how many rak`at he has prayed. So if this happens to anyone of you, he should perform
two prostrations of Sahu while sitting.