The total number of S_ N 2 (nucleophilic substitution bimolecular)...

Total number of $\mathrm{S}_{\mathrm{N}} 2$ reactions involved in the reaction is three. In first $\mathrm{S}_{\mathrm{N}} 2$ reaction methoxy group is replaced by $\mathrm{OH}$ group (i.e. $\mathrm{CH}_3$ group replace by $\mathrm{H}^{+}$of $\mathrm{HI}$ ). In second $\mathrm{S}_{\mathrm{N}} 2$ reaction another methoxy group get replaced by $\mathrm{OH}$. In third $\mathrm{S}_{\mathrm{N}} 2$ reaction $\mathrm{OH}$ get replaced by iodide group.