I've heard the stories that putting SE reputation in the CV can make it look better. I wonder if it is true when it comes to applying for jobs. Is it appropriate to do so? If it is true, how much reputation says that I am good, and how much would suggest that I am a rookie?
Reputation is basically meaningless. Questions that get ranked high in Google search results and/or have click-baity titles will attract more attention and thus get more upvotes. It says little about the quality of the answers or the qualifications of their authors. If you have created some content that you're proud of, you might consider including a link to your profile so that prospective employers can see it but you should also keep in mind that having an inordinate amount of SE content could make people suspect that you have a habit of spending too much company time on non-work-related activities.
On the other hand, it shows that you're a benevolent person who enjoys helping people, which of course would be a good thing. I guess it really comes down to the individual reviewing your resume and how they choose to look at it. Regardless, I highly doubt that just putting your reputation score without any context would be helpful.