The largest chariot battle was the Battle of Kadesh (Qadesh) on the Orontes River in modern-day Syria in 1299BC, with a total of approximately 5,000 chariots. Egyptian Pharaoh Ramses II sought to recapture Kadesh with 2,000 chariots and 16,000 infantry from the Hittite army under Muwatallis, who commanded an infantry of 20,000 with 3,000 chariots. Both sides claimed a victory, with peace being eventually secured in the treaty of 1283 BC.
Please note that the precise date for the battle is unknown, but is usually dated to 1299BC or 1285BC.
Good photo of wall carving: http://en.wikipedia.org/wiki/Image:Ramses_II_at_Kadesh.jpg