I was wondering that if I make my course free, get a few hundred students with free course and then make it paid! I feel like no-one will buy a course with 0 students and 0 reviews!
I would strongly recommend you do not make your course. The only sign-ups you will get will be from automated bots that constantly look for free courses and then signup. These are not real students and will not go through your course. I don't know why this happens, but it does. And, then, even if you do get a few real signups, Udemy will not permit you to send promotions to them. So, there isn't any real advantage to offering your course for free.
But why would people just randomly buy a course with 0 enrolments and reviews!