Marketing tells us that most of the time, higher numbers are better. We want more gigahertz, gigabits, megapixels and milliamp hours, in a package with a smaller number in the thickness. We want our octa-core processors clocked at very high clock speeds with more memory than our former desktop computer and we want it running cooler than a cheesecake... even if most of the time, most of those processor cores are inactive and those that are doing something, are not running at their maximum clock speed. The optimum number of processor cores for the average current mobile operating systems (and through some interpretation, likely desktop machines too) is three. Dual core systems can occasionally bog down, quad core systems have a redundant core some or all of the time. Unfortunately, there are no triple core mobile class processors that I'm aware of: many manufacturers have gone down the quad core route. Hence why we see the Qualcomm Snapdragon 400, 600 and 32-bit 800-series adopting quad core processors. Many, but not all, octa core processors are designed around the big.LITTLE methodology, which combines a high-performance quad core processor with a less powerful, but battery friendly quad core processor. Notable exceptions include the new Nvidia Tegra X1, which uses any core it feels like, and a number of MediaTek processors, which have eight same-design cores in an octa core arrangement.
Now, we have rumors that MediaTek are to release deca-core (ten) and dodeca (twelve) core processors later in the year. If MediaTek are driven by their marketing department, by their customers (in turn driven by their marketing departments) then this would seem to be perfectly legitimate. After all, pull out that old, 32-bit dual core processor and wire up a 64-bit, twelve core processor into any given device and it'll run so fast it will be back in time to smoke kippers for breakfast? No, not quite, but from a marketing perspective it could be something of a win.
You see, the problem with using more and more processor cores is that software needs to be written to take advantage of the extra resource. Android takes some of this load on for developers, but unless a particular device is running many applications at once, much of the time most of those processor cores are detracting from the experience. Those cores require electronic maintenance in the form of a dedicated power management subprocessor; they'll use a tiny amount of power and of course, they make the product more complicated to manufacture and so more expensive. MediaTek are reckoned to be calling their new processors "the bomb," but I'm hoping that they're introducing something else to the mobile processor party other than more cores.