It is equal to Avogadro's number (NA), namely 6.022 x1023. If we have one mole of water, then we know that it will have a mass of 2 grams (for 2 moles of H atoms) + 16 grams (for one mole O atom) = 18 grams. Since we only have 0.001 grams, that means that we have NA * (0.001/18) = 3.35*1019 molecules of water!
