The oxidation number of sodium is +1.
Explanation: Sodium is an alkali metal found in Group 1 of the periodic table. Elements in this group have one electron in their outermost shell, which they tend to lose in chemical reactions to achieve a stable electron configuration. When sodium loses this single electron, it forms a Na+ ion. Since the loss of one electron results in a +1 charge, the oxidation number of sodium is +1.