If $\mathrm{NaCl}$ undergoes complete dissociation in water, the van't Hoff factor $i$ is:

$\mathrm{NaCl}\to \mathrm{Na^+ + Cl^-}$. Each formula unit gives two ions, so $i = 2$ for complete dissociation. The actually observed $i$ for $\mathrm{NaCl}$ in dilute solution is slightly less than $2$ due to interionic interactions.