In the last fifty years of data recorded on Retrosheet, the most in a game was by Stan Williams on May 17, 1961. He threw 207 pitches.
However, considering how pitchers were used historically before then, there is a good possibility that someone has thrown more, just not in the Retrosheet records.
Leon Cadore or Joe Oescherger from 1920 26 inning tie game would be the best candidates. For some reason 303 pitches for one of them sticks in my head.