Suppose an organism can alternate between aerobic and anaerobic respiration. if it had to use anaerobic respiration exclusively, how many glucose molecules must it break down to generate the same atp as it would in aerobic respiration?

When one glucose is broken down under aerobic condition, 38 ATP are normally produced. When one molecule of glucose is broken down under anaerobic condition, 2 ATP are always produced.
If the organism is using aerobic respiration, it will generate 38 ATP. Under anaerobic condition, the organism have to break down 19 glucose molecules in order to produce 38 ATP, since it can only get 2 ATP from each glucose molecule.