[Chris_Ely]

Mecanum wheels have rollers positioned at 45 degrees from the axis of rotation. This transfers the force from the rotation of the wheel by the motor 45 degrees from the axis of rotation. When you power each wheel independently and vary the speed and direction the wheels rotate, you can achieve omnidirectional motion. So, to answer you first question, you need 4 wheels (2 left, 2 right) to drive forward, backward, sideways, and anything in between. The rollers should be oriented such that they form an X when you look from above. If you search "mecanum" in the white papers section, you will find more in depth explanations and example code.