Not as much torque as possible.
You want enough torque to keep the speed constant, regardless of load variations. However, having that, there’s no point in feeding more torque, it would only potentially create additional vibration or noise.
So, speed is monitored several hundreds of time / second, and when a speed deviation is found, more (or less) torque is applied.
It’s important to note that this kind of speed control is very different from the crude systems employed by manufacturers in the 80’s ("Quartz control" do you remember ?). Algorithms are now much more sophisticated (courtesy of the tremendous computing power available today), and they try to "approach" the correct speed smoothly, without over or under-shooting.
