The density of water is 1g/mL.  Assuming that there are no interspaces between water molecules in liquid water, the volume of a water molecule is

$\text{density = }\frac{mass}{voulume}$

Volume of 18gm of water = 18 ml

18gms of water contain ‘N’ number of molecules

So, 18ml of water also contain ‘N’ number of molecules

18 ml water contains $6.023\times {10}^{23}$ molecules

$\therefore$ X ml  contains  1  molecules

$X=\frac{18}{6.023\times {10}^{23}}$

$=2.988\times {10}^{-23}ml$

$=3\times {10}^{-23}ml$