Rotor spin does need to have something providing a balancing force if you don't want to spin. That's why helicopters have tail rotors and quads have two CW props and two CCW props. That said, before quads became the standard design there was a lot of experimenting with tricopters, two motors that have opposing spin directions at the front and the third motor mounted on a "tail boom" that could be rotated by a servo to tilt the propeller to create the forces to not rotate and keep a heading or to turn the craft. So not all "drone" designs have an even number of props.