Total number of resonating structures possible for ${\mathrm{SO}}_4^{-2} \mathrm{ion}$ is 

Structure of 

In different resonating structures of ${\mathrm{SO}}_4^{-2}$, the double bonds present with (1) and (2), (1) and (3), (1) and (4), (2) and (3), (2) and (4), (3) and (4) oxygen atoms. So total possible resonating structures 6