What should a worshiper do if he forgets the number of rak'ahs he has prayed? Thank you.

If a worshiper doubts the number of rak'ahs (units of prayer), he should base his prayer on certainty, which is the lesser number, and then perform the prostrations of forgetfulness (sujood as-sahw). So, if he doubts whether he has prayed two or three rak'ahs, he should consider it two. If he doubts whether he has prayed three or four rak'ahs, he should consider it three. Then, he performs two prostrations before the tasleem (salutation of peace that concludes the prayer).