Are you asking about the 40 man roster that includes players in the minor leagues or just the 25 man roster for your ML team?
On the 25 man ML roster, most go with 12-13 pitchers and use 5 starters. You can get away with an 11 man pitching staff if several of your starters have high stamina (80+) and/or durability (30+) or if your relief pitchers have good stamina/durability. If you are new to this game I would recommend 12-13 pitchers until you get familiar with how stamina/durability ratings and pitch counts work.