Ancient sources disagree by wide margins: Polybius reports 70,000 Romans killed, 10,000 captured and perhaps 3,000 survivors; Livy records 45,500 infantry and 2,700 cavalry killed. Carthaginian losses are given as about 8,000 (Livy) or 5,700 (Polybius). A modern scholar quoted on the source page questions the scale outright, asking whether Hannibal's 50,000 Carthaginians could really have slaughtered 48,000 to 70,000 Romans.
What would resolve this Archaeological or demographic corroboration of ancient troop and casualty figures, or a reconciliation of the Polybius and Livy traditions by a source not yet identified.
OpenAncient historiography and battlefield archaeologyWikipedia: Battle of Cannae