2 required. One to reference the position of the crankshaft and the other to measure the speed. if either of these fail, the engine will have no spark. easily checked with a sensitive volt meter set to ac current, each device will generate 2.5 volts as the engine is cranked. often these are bent after a clutch is replaced or the plug broken from heat brittleness.
Known as: pulse sender

"944 924s Rev.impulse Sensor Crank Reference Sensor "