Where does the figure of 30-32 ATP come from?

The figure of 30-32 ATP comes from the total energy yield of cellular respiration, which includes glycolysis, the citric acid cycle, and oxidative phosphorylation. During glycolysis, a net of two ATP molecules are produced for every glucose molecule. In the citric acid cycle, acetyl-CoA is further broken down, leading to the production of two more ATP molecules (or the energetically equivalent GTP) per glucose molecule. The majority of ATP production occurs during oxidative phosphorylation, with each NADH and FADH2 molecule contributing to the synthesis of additional ATP molecules.