The 50 detents you feel when rotating the shaft by hand comes from the 50 magnetic poles of a standard 1.8° 2-phase motor. Had it been a 0.9° 2-phase motor you would have felt 100 detents.
Moving the rotor between two "natural resting positions" (the detents) takes 4 electrical steps (the 90° phase shifted signal you use to drive the motor). 4*50=200 steps.

/Henrik.