You are referring to the calendar rhythm method, which is NOT and never has been NFP. NFP is based on reading your fertility signals and recording them on a chart, and has nothing at all to do with how many days into your cycle you are.
In NFP, you are considered potentially fertile when you experience vaginal fluids of a sticky, creamy, or slick & stretchy consistency. This often shows up shortly after the period ends, somewhere around cycle day 7 or so. Cycle day 1 is the first day of menstruation, which I believe answers your question.
When charting, you also take your basal body temperature, to see when your average morning temp rises - called a thermal shift. When you see the thermal shift, you know you have ovulated, and are considered infertile on the 3rd night after the shift.
Relying on a "you're fertile between days 8 & 18" system is asking for a pregnancy, because many women experience periodic long cycles where they might not even ovulate 'till day 18 or 20.....and then where would you be?