This is the equation for burning hydrogen in air: #2H_2 + O_2 -> 2H_2O. How many H and O atoms are shown on the left-hand side of the equation?
Clearly, there are 4 hydrogen atoms.........
Clearly, there are 4 hydrogen atoms, but 2 hydrogen molecules, and 2 oxygen atoms, but 1 oxygen molecule on the left hand side of the equation.
And on the RHS, there must necessarily be 4 hydrogen atoms and 2 oxygen atoms, because it is a balanced chemical equation.
This illustrates the principle of stoichiometry, which reflects the conservation of mass. If you start with 10 g of reactant, AT MOST you can get 10 g of product.