Any units will do. For example, let's use the unit of "dozens":

2 dozen of H2 particles + _ (1, 2, or 3) __ dozen of O2 particles → _ (1, 2, or 3) _ dozen water particles

2 dozen of H₂ particles + 1 dozen of O₂ particles → 2 dozen of H₂O particles.

Explanation:

For the reaction of water formation:

2H₂ + O₂ → 2H₂O.

It should apply the the law of conservation of mass that the no. of reactants atoms is equal to the no. of products atoms.

So, every 2.0 moles of H₂ react with 1.0 mole of O₂ to produce 2.0 moles of H₂O.

So, the represented reaction of the problem is:

2 dozen of H₂ particles + 1 dozen of O₂ particles → 2 dozen of H₂O particles.