How many of the following compound(s) is/are aromatic?

The aromaticity of each compound is determined by checking cyclic conjugation, planarity and Hückel's rule.

The first fused polycyclic system is aromatic because it has a continuous conjugated $$\pi$$-system with $$4n+2$$ $$\pi$$ electrons.

The second compound, cyclooctatetraene, is non-aromatic because it adopts a non-planar tub conformation and therefore does not have continuous planar conjugation.

The third five-membered O-N heterocycle is aromatic. It has two double bonds contributing $$4\pi$$ electrons and one lone pair contributing $$2\pi$$ electrons.

$$\pi\text{-electrons}=4+2=6$$

Thus, it satisfies Hückel's rule:

$$4n+2=6\quad(n=1)$$

The fourth fused benzene-cyclohexene system is aromatic because the benzene ring contains a continuous cyclic conjugated system with $$6\pi$$ electrons.

The fifth $$N^+$$-containing five-membered ring is non-aromatic because the positively charged nitrogen does not contribute a lone pair to the $$\pi$$-system. Hence, only $$4\pi$$ electrons are present.

The sixth compound is non-aromatic because it cannot maintain the required planar continuous conjugation.

Cyclopropene and cyclobutene are also non-aromatic because they do not possess continuous cyclic conjugation.

Therefore, the total number of aromatic compounds is:

$$\boxed{3}$$

Therefore, the answer will be 3.