If Udemy is not charging students for your courses (because you set them as free), they are not going to give you any money.
For the sake of completeness: You could earn money from free courses if they are included in the UFB program, but that is not common for free courses.
@RekhaGarg - If you are getting a good amount of signups on your free course it's because these signups come from bots that look for free courses and then automatically sign up. You can no benefit whatsoever from these signups. They are not actual students. You will never earn any money from them.