In most sports, the rules regarding whether a ball that hits the line counts vary by the specific game being played.
For example, in tennis, a ball that hits the line is considered "in," and the point counts. However, in basketball, if the ball touches the line when shooting a three-pointer, it is counted as a two-pointer.
If you have a specific sport in mind, please specify, and I can provide more detailed information about how that sport's rules handle balls hitting lines.