Metallic oxides are basic in nature, Oxides of non-metals are mostly acidic in nature and some are neutral or amphoteric.
Among the given oxides Na2O is a metallic oxide, and it is basic in nature.
Cl2O7 is a non-metal oxide and it is acidic in nature.
N2O and NO are non-metallic oxides but are neutral in nature.
As2O3 is an Amphoteric oxide. It reacts with both acids and bases.
So, The number of amphoteric oxides is 1.