By their very nature, most motors use round shafts. Often, manufacturers will provide one or more "flats" on the shaft to aid in securing an anti-slip drive coupler or mechanism to the shaft, but equally often, the shaft will simply be round in cross-section.
Generally, drive couplers will be secured to the shaft via set screws through the coupler body. If a shaft has flats provided, the intent is for the set screws to be tightened onto the flat, thus providing a more secure attachment. Many couplers will offer two set screws, located at 90° (to each other) around the coupler's circumference.