Number of ${\mathrm{p}}_{\mathrm{\pi }}-{\mathrm{d}}_{\mathrm{\pi }}$ bonds in ${\mathrm{Cl}}_2{\mathrm{O}}_7$ is 

In Cl₂O₇, each Cl is sp³ hybridization and forms three double bonds with three O atoms. So, in total, there is a total of 6$\pi$ bonds and all are ${\mathrm{p}}_{\pi }-{\mathrm{d}}_{\pi }$  bonds.