Some helicopters have on rotor on a horizontal axis perpendicular to the helicopter body at the tail Why is a single rotor never used?

because the spinning motion of the main rotor crates a force called torque, with makes the helicopter's body want to spin in the opposite direction. The tail rotor is used to create anti-torque and stop the body from spinning. In the cockpit, the pilot will use anti-torque pedals which change the angle that the tail-rotor's are spinning at. Allowing the body to spin slightly means it can change direction, and forcing it to spin more in the opposite direction allows it to turn (or yaw) the other way.

A single rotor helicopter will spin uncontrollably in the air and will possibly not take off at all.