Oxidation number of Fe in Fe₃O₄ is:

Concept: 

• The oxidation number/ oxidation state of an element in a compound is the number of charges (positive or negative) assigned to an atom in a molecule or ion according to a set of some arbitrary rules.

Rules for assigning Oxidation number:

• The oxidation number of all elements in the free state is zero. 
• The most electronegative element, Fluorine has a -1 oxidation number in all compounds.
• The oxidation number of monoatomic ions is the same as its charge. For example:
• ​Oxidation state of Li+ , Ca2+ and Al3+ are +1, +2 and +3 respectively.
• The oxidation state of  Cl-  and SO42-  are -1 and -2 respectively.
• Hydrogen has a +1 oxidation state in all compounds except ionic hydrides (-1 oxidation state).
• The oxidation state of oxygen is -2 in a compound except in peroxides which are -1.
• Alkali metals (Li, Na, K, etc.) always have +1, and alkaline earth metals (Be, Mg, etc.) have +2 oxidation states respectively. 

Calculation:

• Let the oxidation number of Iron in Fe3O4 is 'x'.
• The oxidation number of oxide is -2.
• The oxide is neutral as a whole, so,

3x + 4 × (-2) = 0

or, 3x - 8 = 0

or, 3x = 8

or, x = 8 / 3

Hence, the oxidation number of Fe in Fe3O4 is 8 / 3.