15, though I must admit that I do find the apparent equivalence of introversion with Autism amusing in the utmost. I'd probably score 0 without the introversion questions in there.
It's a matter of correlation, and also the idea of being on a spectrum that moves beyond the disease model. In some important ways - particularly response to levels of sensory and social stimulation - autism seems to be associated with an extreme form of what many introverts experience.
I don't really see the association, though that's just my own experience. While I have difficulty engaging strangers in conversation, I consider myself to have very good social skills otherwise and have never heard any differently. Still, I've always weighed heavily towards the introverted side, because I just like to have time to myself.