Amphoteric oxides are the oxides that behave as both acidic and basic oxides. Amphoteric Oxides have features of acidic as well as basic oxides that neutralise both acids and bases.
Acidic oxide: Cl2O7,SiO2,N2O5
Neutral oxide: CO,NO,N2O
Amphoteric oxide: Al2O3,SnO2,PbO2
Hence the answer is 3