As far as I know there is one sensor per wheel. Cause I'm lazy I'll just copy from the bentley manual
If the ABS warning indicator comes on a visual inspection of the system may help to locate the fault. Carefully inspect the wiring harness. Check especially the harness and connections near each wheel for chafing or damage due to incorrectly routed wires. If the wheel sensor harness is damaged, the wheel sensor with its integral harness should be replaced
The pulse wheels themselves may be responsible for faulty speed signals reaching the control unit. Inspect the pulse wheels for damage such as cracks or missing teeth. Check to see that a crack has not made the pulse wheel loose. Spin the tire to check to see if the pulse wheel is out-of-round. Check for dirt or worn wheel bearings which could reduce the clearance between the sensor and the wheel. Even accumulated metallic brake pad dust has been known to cause problems. A faulty pulse wheel should be replaced