Hi @michalska.sylwia14 
Welcome to the GitLab Community Forum!
As far as I know, there’s no way to fetch >20 items from an RSS feed. If you add the RSS feed to a feed reader, it’ll initially fetch the 20 latest items and then continually update after that, so using an RSS feed reader might be a good option if you want to create an archive of items moving forward.
For comprehensive historic data (like issues), you’d be better off using the API so you fetch >20 items and use pagination. For example, to fetch all open issues from the gitlab-org/gitlab project, you could use the following command (which uses glab CLI to handle pagination)
glab api --paginate projects/278964/issues?per_page=100
