How many valence electrons are transferred from nitrogen atom to potassium in the formation of the compound potassium nitride?

            3 electrons are gained by one nitrogen atom in the formation of K₃N.

Explanation:

The reaction is as follow,

                                       6 K + N₂    →    2 K₃N

Oxidation of K,

                                       K     →     K⁺  +  1 e⁻
Or,
                                    6 K     →    6 K⁺  +  6 e⁻

In oxidation reaction potassium atom transfers one electron per atomn and six electrons per six atoms.

Reduction of N;
                                  N₂  +  6 e⁻    →    2 N³⁻

In reduction reaction one molecule of N₂ gains six electrons and 3 electrons per one atom