Dusty Baker - 25
Ricky Henderson - 21
David Ortiz - 20
As for what a normal number should be? No clue. The Sox have had 20 walk offs in the last 4 years. If we then assume about 5 walk offs per year, then Pedroia would have played in about 50 walk offs. 50 walk offs/9 batters = 5 expected walkoffs per batter.
Ortiz played for 15 years, so he would have had an expectation of 8 walk offs. Did a little better though.