2 molecules of phosphoric acid -

one molecule of ${\mathrm{H}}_2\mathrm{O}⟶\left(X\right)\stackrel{+1\text{ (Oxygen atom) }}{\to }\left(Y\right)$

The number of $\mathrm{\sigma }$-bonds in compound Y is :

The reaction is,

$2\times {\mathrm{H}}_3{\mathrm{PO}}_4-{\mathrm{H}}_2\mathrm{O}\to \left(X\right){\mathrm{H}}_4{\mathrm{P}}_2{\mathrm{O}}_7\stackrel{+\left[0\right]}{⟶}{\mathrm{H}}_4{\mathrm{P}}_2{\mathrm{O}}_8\left(Y\right)$

There are total 11 sigma bonds while 2 double bonds presents. Thus, the total bonds,

$$\begin{gathered} =11+2 \\ =13 \end{gathered}$$