The net gain of ATP in anaerobic breakdown of a molecule of Glucose is

When a molecule is broken down anaerobically whatever NADH formed in Glycolysis are used for the formation of either lactic acid or alcohol in fermentation. The total ATP formed in Glycolysis is 4 ATP. Of these 2 ATP are used in the 1st and 3rd reactions of glycolysis. Hence the net gain of ATP is 4 – 2 = 2 ATP.