This figure comes from Pope Gregory II's contribution to Liber Pontificalis which recounts how the forces of Odo the Great, Duke of Aquitaine from ~688-735, had killed 375,000 Arabs/Berbers. This figure comes from Odo, not a third party observer. We know the Arab force was slaughtered but the figure itself may be inaccurate.

See Paul Fouracre's The Age of Charles Martel.