Does the Prophet's saying, "Whoever fasts Ramadan and then follows it with six days from Shawwal, it is as if he has fasted for an entire year," necessitate fasting a full six days from Shawwal, or is it sufficient to fast five days if there is doubt?

It appears that the questioner is in doubt whether he fasted six days of Shawwal or only five days, and it is prescribed for him to fast one more day. This is because the default assumption is not having fasted until it is confirmed with certainty. And whoever doubts whether he has fasted or not, he should fast.