9 cubes should be fine. Might be able to go a little smaller if running 10k. Maybe go with around 8 after displacement. Not sure what cube is the sweet spot for those subs but maybe someone who has used them can help you out.
Have you tried contacting ampere audio and asking them what they recommend for an enclosure for the 12"? its not listed on their website. but it has a very low QTS and their site says they can be used in small vented enclosures.