The scale is different for every guy. But generally, a "9" is the equivalent of the hottest girl you know in real life. A "10" is celebrity/model-hot - absolute physical perfection (A-list Hollywood actresses and Victoria Secret models).
Easy. 10 doesn't exist, or at least I haven't met her and don't think I will. No one is perfect, so I don't give out that score.
I do however give fractional scores, so the difference between 9 and 9.1 would be a harder question...