What is the ruling on one who forgets how many units of prayer he has performed while he is praying?

If you doubt the number of rak'ahs, build upon the minimum you are certain of, then complete the prayer and perform two prostrations (of forgetfulness) before the salutation. However, if this doubt occurs daily (becomes habitual) for you, then build upon the maximum and perform two prostrations to spite Satan. To treat waswas (obsessive whispering), seek refuge with Allah from the accursed Satan and spit lightly to your left three times.