What happens to an enzyme after it breaks down a starch molecule?
Enzymes are a form of catalyst. Catalysts do not get consumed in a chemical reaction. I believe in reality, enzymes do degrade overtime and you need to replenish them overtime to keep the reaction efficient.
Can one (1) enzyme break down multiple starches? Sure.
I did not find specific literature on fermentation, but I found one (1) carbonic anhydrase in our tissues and bloodstream can hydrate 1,000,000 carbon dioxide molecules per second. So your enzyme probably has a similar magnitude of catalytic rate.
Reference
Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Section 8.1, Enzymes Are Powerful and Highly Specific Catalysts. Available from: https://www.ncbi.nlm.nih.gov/books/NBK22380/