4 stroke radials have odd numbers of cylinders to keep the firing equally space - i.e. for a 7 cylinder: 1-3-5-7-2-4-6 back to 1....)
An even number of cylinders results in 2 cylinders firing beside each other, introducing an uneven power pulse and vibrations (1-3-5-2-4-6 ->back to 1, 1 and 6...