1. List all the given oxides

We need to count acidic oxides among: $\mathrm{N_2O_3,\ NO_2,\ N_2O,\ Cl_2O_7,\ SO_2,\ CO,\ CaO,\ Na_2O,\ NO}$

2. Recall the idea of acidic oxide

Acidic oxides are generally non-metal oxides that react with water to form acids or react with bases to form salt and water.

3. Check each oxide one by one

(i) $\mathrm{N_2O_3}$

It is the anhydride of nitrous acid: $\mathrm{N_2O_3 + H_2O \rightarrow 2HNO_2}$ So, $\mathrm{N_2O_3}$ is acidic.

(ii) $\mathrm{NO_2}$

It gives acids with water: $\mathrm{2NO_2 + H_2O \rightarrow HNO_2 + HNO_3}$ Hence $\mathrm{NO_2}$ is acidic oxide.

(iii) $\mathrm{N_2O}$

This is a neutral oxide.

(iv) $\mathrm{Cl_2O_7}$

It is the anhydride of perchloric acid: $\mathrm{Cl_2O_7 + H_2O \rightarrow 2HClO_4}$ So it is acidic.

(v) $\mathrm{SO_2}$

It forms sulphurous acid with water: $\mathrm{SO_2 + H_2O \rightarrow H_2SO_3}$ So $\mathrm{SO_2}$ is acidic.

(vi) $\mathrm{CO}$

This is a neutral oxide.

(vii) $\mathrm{CaO}$

This is a basic oxide.

(viii) $\mathrm{Na_2O}$

This is a basic oxide.

(ix) $\mathrm{NO}$

This is a neutral oxide.

4. Count the acidic oxides

Acidic oxides are: $\mathrm{N_2O_3,\ NO_2,\ Cl_2O_7,\ SO_2}$

Total number of acidic oxides: $4$

5. Compare with stored correct answer

Stored correct answer = $4$

Our derived answer matches it.