I believe our intrepid intial poster is referring to the number of cylinders used. While the math may be easier, ain't nobody got time for no even-numbered cylinder radial...
Yes, but the Curtiss Challenger was a two-row engine, and two-row radials with even cylinder numbers are common. Much less common are four-stroke radials with even numbers of cylinders per row. The 12-cylinder Curtiss H-1640 Chieftain is an example of one such "hex" engine. Later, Curtiss-Wright experimented with a larger hex engine, the H-2120. These are discussed in more detail at http://www.enginehistory.org/Piston/Wright/wright_aero.shtml#Hex. The British Bristol company built a 16-cylinder "Hydra" with 8 cylinders per row.
By the way, the engine pictured at the top of this thread would work fine as a two-stroke.