Napier Deltic T18
The Deltic is an opposed-piston, 2-stroke, 3,300 HP railroad and marine engine which began life as a Junkers aircraft engine concept. Deltics have 3 crankshafts, 18 cylinders, 36 pistons.
Here's a Deltic T9 starting. The T9 has only 9 cylinders:
A T18 running at full rack: