Coefficient in front of molecules in a chemical equation
There's the trick: it's all of them.

The coefficients are a ratio. For every two unit of hydrogen molecule, you need one unit of oxygen molecule, and the result is two unit of water molecule.

That unit is often the particle itself, whether it's an atom, a metal ion, a more substantial molecule or even a protein or enzyme. It can also be the volume (for ideal gases under the same conditions), the moles used in reaction, or molarity in solution.

Simply remember that the coefficient applies to the item following it as a whole. As long as the ratio between coefficients is the same, you can use any unit that maintains it.