Twelve stitches per inch seems excessive to me and, in my very humble opinion, would look too much like machine quilting. I am quite happy with my 4 to 5 stitches per inch.
Again, strictly my opinion, but consistency is far more important than the number of stitches. I saw a quilt yesterday that had only 3 or 4 stitches per inch (I wasn't really counting, so I'm guessing) but every stitch was dead on consistent. It was fabulous to look at and made me drool with envy. That's the perfection I strive to achieve.