The sensors are basically read in parallel, so it shouldn’t take significantly longer to read 13 sensors than it would to read 8.
With a well-tuned line follower, even 2 sensors (one located on either side of the line) could be enough to follow a line tightly; however, more sensors can give you more information. For example, if your course has sharp 90-degree turns, it is useful to have more sensors further away from the line to help detect and identify those.
Brandon