The number of amphoteric oxides among Al2O3, ZnO, CO, N2O, SnO2, BeO, PbO and CaO is

Amphoteric oxides react with both acids and bases: $\mathrm{Al_2O_3}$, $\mathrm{ZnO}$, $\mathrm{SnO_2}$, $\mathrm{BeO}$ and $\mathrm{PbO}$, five in all (metal oxides near the metal/non-metal border, plus the small $\mathrm{Be^{2+}}$ ion). $\mathrm{CO}$ and $\mathrm{N_2O}$ are neutral oxides, and $\mathrm{CaO}$ is clearly basic (it reacts with water to form $\mathrm{Ca(OH)_2}$ and does not dissolve in alkali). The answer is therefore $5$.