Must one stop the prayer and start anew when in doubt about the number of rak'ahs, or should the worshiper continue their prayer?

Doubt in prayer does not invalidate it and is of two types: The first is when one of the two possibilities is more likely, so the worshipper builds upon what is more likely, completes their prayer, performs the salutation, and then prostrates for forgetfulness. The second is when neither of the two possibilities is more likely, so the worshipper builds upon the lesser (number of units), completes their prayer, and prostrates for forgetfulness before the salutation.