The number of water molecules is maximum in

$18 \mathrm{grams}$ of water corresponds to $1 \mathrm{mole}$ and contains $=6.02\times {10}^{23}\mathrm{ }$water molecules.
$18 \mathrm{moles}$ of water contains$=18 \times 6.02\times {10}^{23}\mathrm{ }$ water molecules.  
 $18$ molecules of water
 $1.8 \mathrm{grams}$ of water contain $\frac{1.8}{18}\times {\mathrm{N}}_{\mathrm{A}}=0.1{\mathrm{N}}_{\mathrm{A}}$ water molecules.

So, $18 \mathrm{mole}$ water has a maximum number of molecules.