The number of water molecules is maximum in 

$1.8\text{ gram of water }=\frac{6.023\times {10}^{23}}{18}\times 1.8$

${\text{=6.023×10}}^{22} \mathrm{molecules}$

$18 \mathrm{gramofwater} =6.023\times {10}^{23} \mathrm{molecules}$

$18 \mathrm{moles} \mathrm{of} \mathrm{water} =18\times 6.023\times {10}^{23} \mathrm{molecules}$