In one mole of H2O, there are exactly 6.022 x 1023 molecules of water. This specific number is known as Avogadro’s number, which is the number of atoms, ions, or molecules in one mole of a substance.
To explain further, when we say ‘one mole of H2O’, we are referring to the amount of water that contains the same number of entities (molecules, in this case) as there are in 12 grams of carbon-12. This constant allows scientists to convert between the microscopic scale of atoms and molecules and the macroscopic scale that we can measure in the lab.
Thus, regardless of the type of substance, the mole provides a linkage to understand the quantity of particles in a given mass. So, whenever you have one mole of any substance, including H2O, you will always have 6.022 x 1023 of its molecules.