It's always interesting to see how a somewhat arbitrary round number like 100 becomes the keystone for a starting pitcher. Maybe it's equivalent for a reliever is 20 pitches per 3 outs. I'm sure the team data analysts have individual numbers based on past performance for each pitcher and his relative effectiveness, as the overall count goes higher . Observationally, it seems if you can get a pitcher over 25 pitches in any one inning, good things happen on the scoreboard for the offense.
The other thing about the 100 pitch milestone is that the starter's pregame routine includes a long toss sequence, 25-30 bullpen pitches, and 8 warm ups from the mound each inning. Of course, the warmups aren't under full stress, but a starter who goes 6 full innings has probably thrown 100 pitches at full effort, and another 100 at 80-90% effort including the 120'-130' long toss session.