If a plane crashes because of a sensor. You need 50 or more depending on MTBF. crazy. Benefit vs loss calculation.
You wouldn’t need 50. You just need enough to determine that there is a fault so the pilots can be aware of the situation. With 3 sensors, 2 sensors can outvote the 1 defective sensor. If a fight has an issue with a faulty sensor, the plane should not take off again till the sensor is replaced. In the Lion Air case, the same plane had problems with the same sensor on multiple flights, but the Lion Air maintenance did not replace the sensor after any of those flights.