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.