The complete beta oxidation of a C20 fatty acid involves the step-by-step breakdown of the fatty acid molecule into acetyl-CoA units. Each round of beta oxidation involves four key steps: oxidation, hydration, oxidation, and thiolysis. Let's break down the process and calculate the number of ATP molecules generated.

1. Oxidation
In the first step, the fatty acid is oxidized by a flavoprotein dehydrogenase, producing a trans double bond between the alpha and beta carbons of the fatty acid. This reaction is catalyzed by acyl-CoA dehydrogenase. One FADH2 molecule is produced during this step.

2. Hydration
In the second step, water is added across the double bond, resulting in the formation of a beta-hydroxyacyl-CoA molecule. This reaction is catalyzed by enoyl-CoA hydratase.

3. Oxidation
In the third step, the beta-hydroxyacyl-CoA molecule is oxidized to form a beta-ketoacyl-CoA molecule. This reaction is catalyzed by beta-hydroxyacyl-CoA dehydrogenase. One NADH molecule is produced during this step.

4. Thiolysis
In the final step, the beta-ketoacyl-CoA molecule is cleaved by thiolase, resulting in the release of acetyl-CoA and a shortened fatty acid chain by two carbon atoms. This process generates one molecule of ATP, one molecule of FADH2, and one molecule of NADH.

Since the C20 fatty acid undergoes nine rounds of beta oxidation to completely break it down into acetyl-CoA units, we need to multiply the number of ATP molecules generated in each round by nine.

Calculation:
1 FADH2 molecule is generated in each round of beta oxidation, so 9 FADH2 molecules are produced.
1 NADH molecule is generated in each round of beta oxidation, so 9 NADH molecules are produced.
1 ATP molecule is generated in the thiolysis step of each round of beta oxidation, so 9 ATP molecules are produced.

Therefore, the total number of ATP molecules generated from the complete beta oxidation of a C20 fatty acid is 9 ATP.

However, it's important to note that this calculation does not take into account any ATP used in the activation of the fatty acid or the transport of the acetyl-CoA into the mitochondria. These processes require an additional expenditure of ATP.