The number of $P-OH$ bonds and the oxidation state of phosphorus atom in pyrophosphoric acid $\left(H_4{P}_2{O}_7\right)$respectively are:

The number of P−OH bonds in pyrophosphoric acid $\left(H_4{P}_2{O}_7\right)$ is four.

The oxidation state of the phosphorus atom in pyrophosphoric acid $\left(H_4{P}_2{O}_7\right)$:

$$\begin{gathered} 4\left(\mathrm{oxidation} \mathrm{state} \mathrm{of} \mathrm{H}\right)+2\mathrm{x}-2\left(\mathrm{oxidation} \mathrm{state} \mathrm{of} \mathrm{O}\right)=0 \\ 4\times 1+2\mathrm{x}-2\times 7=0 \\ \mathrm{x}=+5 \end{gathered}$$