I want to know how many users in total are visiting my page. But I have read that the total users don´t need to be the real ones, because we can have some new users that return later several times.
So, were can I find the exact number of REAL UNIQUE users that visit my page in total? Or how can I make the operation in order to get this result?
There are a few things to know to understand the User numbers.
A "User" is determined by a cookie set by GA. Two people sharing a computer and using the same browser will count as one User, and conversely one person using two different browsers/devices, or clearing their cookies between two visits, will count as two Users. If visitors are typically logged in to your site when they are using it, you can set up User ID Tracking, where you set a cookie with some unique identifier for the account when the person is logged into it (it is against Google's terms of service to have the User ID cookie set for logged-out visitors, even if you know they are the same person). In that case a person who logs into their account from two different devices will be counted as a single User.
A "New User" in a given date range is a user who had their first session during that time. "Returning Users" are the users who had their second or later visit during that time; there are no restrictions on when their first visit happened. Returning Users can overlap with New Users: if a user's first and second visits both happened during your date range, that user will count in both groups. The total User number counts each user only once, however, no matter how many times they visited in your date range. This is the number that you are looking for, with the understanding that it is not exact -- both because of the limitations above of how GA understands what a "user" is, and because web tracking is not perfect and can also be blocked.
Final notes for completeness: In the Audience > Behavior > New vs Returning report, there is no overlap, because it is counting sessions that are first-time visits versus sessions that are not first-time visits. Likewise the system segments "Single-Session Users" and "Multi-Session Users" do not overlap, because they count users according to whether they had one or more sessions during your date range, whether they are new in one of those sessions or not.
