I changed my rear brakes the other weekend and I still get the brake wear notification. How many brake pad wear sensors do I need for my car, front and rear?

I have a 2003 E320 sedan, base model. I notice when changing the rear brakes, left side only had the sensor. Did I lose the other side? Haven't changed the fronts yet because the pads still look good, about 70% wear...

Thanks!

IIRC....Fronts have a sensor on each side. Rears only have one sensor.