n-factor for ammonium nitrite?
"n factor" of oxidizing or reducing agent = total number of electrons transferred per a mole of the reactant.

When more than one element in a substance is oxidized/reduced, the n-factor of the substance is taken as the sum of the n-factors of the individual elements of the substance.

This is the case here.You have  ammonium ion that undergoes oxidation reaction (n-factor= 3) and  nitrite ion that undergoes reduction reaction (n-factor= 3).
So, the n-factor of ammonium nitrite is 6.