In cellular respiration, one molecule of glucose, which contains 6 carbon atoms, reacts with oxygen. Since respiration is represented as a balanced chemical equation, the total number of carbon atoms on the reactant side is the same as the total number of carbon atoms on the product side.

$$C_6H_{12}O_6 + 6O_2 → 6CO_2 + 6H_2O $$

Therefore, there are 6 carbon atoms in the reactants of respiration.