Number of $\mathrm{S}-\mathrm{S}$ linkages in ${\mathrm{S}}_2{\mathrm{O}}_7^{2-}$.

The structure of ${\mathrm{S}}_2{\mathrm{O}}_7^{2-}$ is given below.

Here, S-O-S linkage is present but no S-S linkage is shown. Thus, the number of $\mathrm{S}-\mathrm{S}$ linkages in ${\mathrm{S}}_2{\mathrm{O}}_7^{2-}$ is 0.