The length of your course depends entirely on your subject. I have a course that is fourteen hours, there are courses of more than thirty hours on Udemy, and some as short as 30 minutes.
But.... the real issue in your question is that you asked "tell me, MINIMUM how many....,"
Why do you want to know the minimum? Do customers buy the minimum? Are you going to compete against other instructors in your subject by providing the minimum? If you do you will have minimum success on Udemy. You succeed by providing the maximum value. Look at the best selling, highest rated, courses in your subject area and ask yourself "how can my course be better, provide more value than those courses?"