A Sensor Array always has 8 Sensors, after all, the rules state that explicitly.
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Sensor array: This sensor package includes up to eight functions... I imagine that regular vehicles only have what sensors are required for grid guide; cameras, atmosphere sensor, laser rangefinders, and a microphone for voice commands. Your average Americar doesn't need anything else. Clearly, a rigger will want to tweak them out and install their own sensors.
Drones dont need anything else for navigation either. Certainly, specialized drones will include more sensor functions.
As far as sensor ratings and armor, if you can fit it into a cyberlimb, you can build it into your armor. I cap armor sensors at rating 5. I believe Run&Gun suggests your can add sensors to armor under the armor accessories section, but im away from my books. The head wear ultrasound cyber gives you up to rating 6, even, going one step beyond the sensor rules. Theoretically a cyber arm could hold two full sensor arrays.