In Major League Baseball, the standard pitching rotation consists of five pitchers. Too few pitchers and you won’t have enough rest, leading to injury and underperformance. Too many pitchers and you end up giving starts that would have gone to your best pitchers to your worst.
With that being said, there are two factors that complicate the five-man rotation. First, that every team in baseball uses more than five starting pitchers in the course of a full season. In 2019, 10 different pitchers started a game for the Royals, and seven of whom made at least nine starts; lest you think that figure was marred by the Royals being awful, the 2015 Royals also used 10 different starters, seven of whom made at least nine starts.