Answer is: the total increase in oxidation number of the species being oxidized in unbalanced chemical reaction is +2, because carbon in carbon(II) oxide (CO) change oxidation number from +2 to +4 in carbon(IV) oxide (CO₂) and in balanced chemical reaction is +6 (3 · (+2)).
Unbalanced chemical reaction: Fe₂O₃(s) + CO(g) → Fe(s) + CO₂(g).
Oxidation half reaction: C⁺² → C⁺⁴ + 2e⁻ /×3; 3C⁺² → 3C⁺⁴ + 6e⁻.
Reduction half reaction: Fe⁺³ + 3e⁻ → Fe⁰ /×2; 2Fe⁺³ + 6e⁻ → 2Fe⁰.
Balanced chemical reaction: Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g).