On April 22, 2018 — Facing rookie Jaime Barria of the Angels in the 1st inning, Brandon Belt of the Giants forces him to throw a record 21 pitches before flying out to right field. The 12-minute battle features 16 consecutive foul balls at one point, in addition to four unsuccessful pick-off attempts at first base. The previous record was 20 pitches, in a 1998 confrontation between Ricky Gutierrez and Bartolo Colon, although detailed pitch counts have only been tracked since 1988, so there may have been longer at-bats before that which went unrecorded.

Source
Baseball Reference April 22