When you select curved auto labeling, MapInfo Pro attempts to create a curved label for every arc/polyline record in a map, just as it does for non-curved labels.
For example, in street maps, the street can be made of several polylines or one long polyline. The length and number of the polylines, the rules that govern whether a curved label can be created, and the labeling options you choose, all affects which curved labels are created and where they display.