Here Fe and O₂ are reactants and Fe₂O₃ is a product. It is a balanced chemical reaction in which the number of atoms on LHS are equal to number of atoms on RHS.
4 atoms of iron react with 3 atoms of oxygen to form iron oxide. Hence, there are 4 atoms of reactants in this equation.
3 to balance the calciums on both sides of the equation, but only if that balances the Cl on both sides of the equation to because you now have 6 Cl and 3Ca
