Total number of acidic oxides among N$$_2$$O$$_3$$, NO$$_2$$, N$$_2$$O, Cl$$_2$$O$$_7$$, SO$$_2$$, CO, CaO, Na$$_2$$O and NO is ______.

We need to count the number of acidic oxides from the given list.

An acidic oxide reacts with water to form an acid, or reacts with a base to form a salt.

Analyzing each oxide:

N$$_2$$O$$_3$$: Acidic oxide (forms HNO$$_2$$ with water). ✓

NO$$_2$$: Acidic oxide (forms a mixture of HNO$$_3$$ and HNO$$_2$$ with water). ✓

N$$_2$$O: Neutral oxide. ✗

Cl$$_2$$O$$_7$$: Acidic oxide (anhydride of HClO$$_4$$). ✓

SO$$_2$$: Acidic oxide (forms H$$_2$$SO$$_3$$ with water). ✓

CO: Neutral oxide. ✗

CaO: Basic oxide. ✗

Na$$_2$$O: Basic oxide. ✗

NO: Neutral oxide. ✗

Total number of acidic oxides = $$\boxed{4}$$