The number of $\pi$ -electrons in the molecules of aromatic compound is

According to Hückel's Rule of Aromaticity, a planar, cyclic, continuously conjugated ring system shows aromatic stability if its delocalized cloud contains exactly $(4n+2)\pi$ -electrons, where $n$ is a non-negative integer ($n=0,1,2,3,\dots$).Common examples include benzene ($6\pi$ -electrons, where $n=1$) and naphthalene ($10\pi$ -electrons, where $n=2$).