How big is a drop?
"A drop" is not a precise measurement. It depends on a number of factors.
If you measure how big the drops of water are you're talking about, (in millilitre)
then you could figure out the number of molecules based on the fact that Avogadro's number of water molecules have a volume of about 18 milliliters.
Avogadro's number is 6.022 * 1023 (particles) per mole
One drop: average volume about 0.05 ml = 0.05 g
Molar mass of water 18.01 g/mol
Meaning: about 1.7*1021 molecules in a drop of water: don't start counting them!!