Maximum number of equal $\mathrm{P}-\mathrm{O}$ bonds in ${\mathrm{P}}_2{\mathrm{O}}_7^{4-}$ ion is_____________

 In phosphate anion, the $\mathrm{P}$ atom is bonded to one oxygen atom with double bond and no charge on that oxygen, and singly bonded to other three oxygen atoms with one unit negative charge on each oxygen atom.
Thus, P has no lone pair of electrons on it. This is the main reason for the almost equal bond lengths of all the P - O bonds because there is no lone pair bond pair repulsion. The bond pairs of P exert equal repulsions on all the bond pairs and lone pairs of O respectively .
Due to this reason, the $6\mathrm{P}-\mathrm{O}$ bonds in ${\mathrm{P}}_2{\mathrm{O}}_4^{7-}$ ion are equal.