FWIW Steinberg inform me that there is no limit to the number of cores Cubase can use. In theory if you build a dual-12 core system, Cubase will see all 24 cores (48 cores with hyperthreading) and use all of them. Whether that will deliver the best performance is perhaps debatable, but word from the horses mouth is there are no limits, and apparently this has been the case since around Cubase 5.