Glavine throws a fastball, circle change, curveball, slider, and added a cut fastball in 2000.
And Rivera has always thrown two pitches. A regular fastball, and a cut fastball. Many, many relievers throw mainly two pitches. Billy Wagner does, as does Francisco Rodriguez.
Relievers only go 1-2 innings tops, so they can throw 1-2 pitches. Starters need a wider variety, since they see each batter 3-4 times a night.