If you cut a regular dodecahedron in half, it looks to me like you can get 10 or 12 faces easily. If you bisect the diagonal from one vertex to the opposite you cut off three of the original faces, leaving 9, and add your new plane for 10. If you cut it parallel to two faces you cut off one pentagon, but get parts of 10 faces, one whole pentagon, and the cut face for 12. I don't think it is half volume, but it looks like you could cut it perpendicular to the line joining the midpoints of opposite edges and cut off four faces plus add your cut for 9.