Among the following oxide of $\mathrm{p}$ - block elements, ${\mathrm{Cl}}_2{\mathrm{O}}_7,\mathrm{CO},{\mathrm{PbO}}_2, {\mathrm{N}}_2\mathrm{O},\mathrm{NO},{\mathrm{Al}}_2{\mathrm{O}}_3,{\mathrm{SiO}}_2, {\mathrm{N}}_2{\mathrm{O}}_5, {\mathrm{SnO}}_2$

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: ${\mathrm{Cl}}_2{\mathrm{O}}_7,{\mathrm{SiO}}_2, {\mathrm{N}}_2{\mathrm{O}}_5$
Neutral oxide: $\mathrm{CO},\mathrm{NO},{\mathrm{N}}_2\mathrm{O}$
Amphoteric oxide: ${\mathrm{Al}}_2{\mathrm{O}}_3,{\mathrm{SnO}}_2,{\mathrm{PbO}}_2$

Hence the answer is $3$