Question: - “How many crank rotations are required on a 12 cylinder 4-stroke engine for each cylinder to fire once?”

The key to the answer to the question is in the question; it is a four stroke engine. In a a four stroke engine, all engine functions (Intake, Compression, Power and Exhaust) happen in two full revolutions of the crankshaft. This cycle happens once and only once for each cylinder in each two revolutions. It doesn’t make any difference if the engine has one or 16 cylinders. All cylinders perform all four (4) strokes in any two full crankshaft revolutions. But not at the same time, but within the same two revolutions. That means that, for example, in a 12 cylinder, four-stroke engine, there are 12 power strokes in any continuous two crankshaft revolutions. In a 6 cylinder four-stroke engine, there are six power stokes in any two crankshaft revolutions. That’s the rule.