Yep, you need one speed controller for each side of your drive. So for 2WD, one speed controller for the left motor and one for the right. If you have 4WD, you still only need one speed controller per side (even though each side now contains two motors - just make sure your speed controller can handle the combined stall current of the motors)
And if you have three sides to your drive (the doom that is Triangulary-bot ) youd need three speed controllers