How many molecules are in 0.001g of KCl?

KCl is an ionic compound which contains K⁺ and Cl⁻ ions.

Hence, 0.001 g of KCl does NOT contain any molecule.

hmmm
.. you joined 4 days ago
.. 4 days ago you posted "stoichiometry" questions. 
.. 4 days ago you posted "molarity questions"
.. 7 hrs ago you posted "relative abundance questions"
.. 7 hrs ago you posted more "molarity questions"
.. 7 hrs ago you posted more "stoichiometry" questions
.. 7 hrs ago you posted "acid base stoichiometry" questions
now you're posting what is that?
.. conversion of mass to moles to molecules (of an ionic compound)
really? 

<sigh>.. are you another TROLL or just the same TROLL that's been menacing YA for months now with a different name?

"8.1e18 molecules"

The correct term to use is 'formula units.' as in:

8.1e18 formula units

This is because there are no molecules in ionic compounds.

KCl is 74.55 g/mol
6.022e23 molecules/mole     Avogadro constant

0.001 g / (74.55 g/mol) x 6.022e23 molecules/mol = 8.1e18 molecules