Find the number of species having two $p\pi -p\pi$ bonds.

${\mathrm{SO}}_2,{\mathrm{SO}}_3,{\mathrm{XeOF}}_4,{\mathrm{XeO}}_2{\mathrm{F}}_2,{\mathrm{POCl}}_3,{\mathrm{CO}}_3^{2-},{\mathrm{NO}}_3^{-}$

None have two $p\pi -p\pi$ bonds. Because there is no molecule has a bonding between two atoms when one atom has an empty orbital and the other has a lone pair of electrons.

If the electron pair is given to the appropriate unoccupied orbital, the bonding p- pi p - pi is produced. 

Depending on the orbital that the electron pair is transferred to and from, the bonding will change.