Walys Flarum doesn't have a WYSIWYG editor built-in, so it will always show the full post source code, which has to include some sort of identifier for which post is being mentioned or replied to.
An extension like https://discuss.flarum.org/d/26455-wysiwyg-rich-text-editor might be able to "hide" it for you in the editor, but that number will always be there behind the scenes in the post that is being submitted. I'm not actually sure if mentions benefit from a rich preview like this in that extension or if they are just rendered as text.
The reason there's a number after simple mentions (not replies) is for support with custom non-ASCII display names. It's the user ID. There's a setting in the Flarum Mentions extension to support the old format with just @username
but it's automatically converted on save. The format Flarum will use moving forward will continue to be based on user and post IDs.
Walys if I undestand correct, the number to references a specific post in the future with some time and a lot of post is go up no? Maybe in Flarum in one year have a 8 , 9 or 10 digits
That's correct. But the incrementing identifiers are unique to each forums. Most small forums won't ever see such large numbers. Here on Discuss the numbers are getting bigger because we have a lot of activity. There's however no storage or performance concern to have with the identifiers being longer.
It's not much different from sharing links to other discussions. Of course the (shortened version of) links will become longer as the number of discussions grow since the identifiers will become longer. But I have never heard of this being an issue on any forum.