According to Official Guinness Records,
The most people throwing a ceremonial first pitch simultaneously was achieved by 111 pitchers and was organized by the Ehime Mandarin Pirates (all Japan) at the Botchan Stadium, Ehime-ken, Japan, on 26 August 2011.
118 pitchers pitched but 7 were discounted. To check the timing of each pitcher 5 video cameras were used, placed all around the stadium.
For a complete list of 2011 records, please visit 2011 Guinness Records in Japan.