The free energy for the oxidation of glucose to CO2 and water is -686 kcal/mol and the free energy for the reduction of NAD to NADH is 53 kcal/mol. What is the main reason that only two molecules of NADH formed during glycolysis when it appears that as many as a dozen could be formed

in the process of  of glucose oxidation, it produces CO₂ and water and releases  -686kcal/mol of free energy and in the reduction reaction of NAD+ to NADH, it produce +53 kcal/mol of free energy. only two molecules of NADH is formed during glycolysis, because-

Most of the free energy available from the oxidation of glucose remains in the pyruvate form which is one of the products of glycolysis.