Are those petrol or diesel?, the only reason for different cylinder counts is when it's ignition based, otherwise there's no need - if it's based simply on engine rotation (as you would expect with a diesel) it's just an arbitrary design choice. But for a petrol engine, the ignition system makes an obvious (and already present) source for tacho signals, and obviously dependent on the number of cylinders, but also on the number of ignition coils (presumably a 6 cylinder dual coil system would only provide 3 cylinder pulses?).