Consider the following compound. 

If the above compound is treated with excess of NaCN (in acetone),how many $C{N}^{-}$ group would be incorporated by $S_{N}2$ reaction ?

Only the circled groups (leaving groups) will be substituted by $C{N}^{-}$ (nucleophile) in $S_{N}2$ reaction
 
Cl (a) (aryl),Cl(b) and I(c) (vinyl) are attached to $s{p}^2$, sp $s{p}^2-$ carbons respectively and they will not be substituted here. 
$\therefore$ Number of circled groups=3