The Thumper does have 6 motors, but each side is wired in parallel:
This is a differential-drive chassis, meaning that turning is accomplished by driving the motors on the two sides of the platform at different speeds. The three motors on each side of the robot are wired in parallel, so only two channels of motor control are required to get this chassis moving. The motors are intended for a maximum nominal operating voltage of 7.2 V (2V minimum), and each has a stall current of 6.6 A and a no-load current of 420 mA at 7.2 V.
At least that is what I read here: https://www.pololu.com/product/1561
I think most of the other tank kits I’ve seen have just 2 motors though.