Two to three dozen is the short answer. The long answer is it depends on the variety, the growing conditions, how consistent you are with watering and how dedicated you are to harvesting. One Black Beauty hybrid zucchini should easily give you two to three zucchini a week for a two to three months harvest season.
So…two to three a week for two to three months gives you two to three dozen zucchini!
Be sure to harvest your zucchini when they are no bigger than 15 centimeters (6 inches) long. Allowing your plant to produce a single zucchini the size of a Volkswagen bug will deplete the plant of energy and make the overall production less. So as fun as it is to pick a fruit you have to carry with both arms, resist the urge! Harvest zucchini when they are small. Your harvest will be longer and taste better too.