Spokespersons are the club's ambassadors. They improve mood of Sponsors, and, to a much smaller degree, mood of fans, thereby resulting in an increase in fan club size and sponsor income. If your team is performing very poorly the spokespersons can only help you reduce the decreases in fan club size and sponsor income.
Research in the newbie (English) conference suggests that optimal number of spokespersons is around 1 per 250 fans. But it might be useful to temporarily hire more than optimal at the start of the season to recover sponsor mood quicker from the reset.
Warning: More than 10 spokepersons will make your sponsors mood drop.