This photograph is of the new rudder box with a NEMA17 stepper (underneath) and the two gears. The smaller gear is on the stepper and the larger, quarter gear will be clamped to the rudder shaft. It's a 4:1 ratio and the rudder gear is 90 degrees so one complete turn of the stepper will bring the rudder from end to end. That's plus or minus 100 steps.
This box will be mounted above the deck, with the rudder shaft disappearing down through the hull.
Rudder angles greater than around 40 degrees either side are counter-productive, as the rudder stock starts to act more as a brake than a steering surface. So a 90 degree swing is plenty.