Your magnet is one pole. Now on a clutch bell you have two magnets in there but that is to balence the bell. One mag. goes in with the North Pole pointed up. The next mag. goes in with the South Pole facing up. The sensor only picks up the North Pole of the mag. Hence one pole. Think of it this way. On a electric motor the number of mags inside the motor is the number of poles you enter in you esc for the gov. Same thing on a nitro. With a backplate sensor you only have one pickup. Hence one pole. If you had a two cylinder motor you would enter two poles for two crank pins. So on and so forth.
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Here are a couple of pics may be help you understand.
Here you can see I have a pointer pointing at the first mag. The other mag. is on the opposite side of the bell. The one mag. is north pointing up. The opposite mag. the south is pointing up.
Here you can see the sensor and the one mag. Has the mag. with the North Pole comes around, the sensor picks it up. When the mag. with the South Pole comes around, the sensor can't pick it up. One again, one pole.