The numbers of electrons delocalised in the pi system of $\mathrm{O}{}_3$ and of $\mathrm{NO}{}_3{}^{-}$ are, respectively,

In ozone the pi system spreads over three O atoms with 4 electrons (one pi bond plus one lone pair of the terminal O). In nitrate it spreads over four atoms with 6 electrons (the N=O pi bond plus lone pairs on the two O${}^{-}$ atoms: $2+2+2=6$).