What should a person do if they are uncertain about how many rak'ahs they have performed in prayer?

Acts of worship and deeds should be free from any doubt. If a person experiences uncertainty for the first time since reaching puberty about how many rak'ahs they have performed in a prayer, they must repeat that prayer. If this doubt occurs from time to time, the person should act according to their predominant opinion (zann-ı galib). If they do not have a predominant opinion regarding the number of rak'ahs performed, then they should proceed by considering the lower number of rak'ahs. (Şürünbülâlî, Meraki’l-felah,182). For example, if someone starts a four-rak'ah prayer and becomes unsure whether they are in the first or second rak'ah and cannot decide, they should assume they have performed only one rak'ah. Since there is a possibility that what is considered the first rak'ah is actually the second, and what is considered the third is actually the fourth, the person should sit and recite the tashahhud at the end of each rak'ah, and finally perform the prostration of forgetfulness (sehiv secdesi) to complete the prayer (Kâsânî, Bedâi‘, 1/165, 166).

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu