How to find number of pπ-dπ bonds in P4O10
In the P4O10 the bonds on the corner are much shorter than a single bond and are in fact double bonds. These bonds are formed by p π-dπ back bonding.
A full p orbital on the O atom overlaps sideways with an empty d orbital on the P atom hence the bond is a dative bond.
Hence it has 4 coordinate bonds formed by pπ-dπ back bonding.