Is the total amount of water on Earth always constant?

The total amount is not exactly constant, as there are two fluxes of water between Earth and the rest of the solar system. There is a steady rain of water-bearing meteoroids hitting the planet, which slowly increases the amount of water. read more

The Earth is effectively a closed system and the total amount water it contains is essentially constant. Now, some of that water is stored in humans temporarily while they're alive. So, the more humans there are then the greater the volume of water that will be stored in that reservoir. read more