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.