For some unknown reason, Atlassian have placed a 5000 issues limit on the Board Backlog. This limitation will count not just your stories, but also the subtasks in each story. If the total number of items is more than 5000 issues, then the backlog will stop working and a warning will be shown instead.
In most cases this will not matter, because 5000 is still a lot of issues in one board, but if you try to setup collection projects that fetch data from multiple projects, this could cause problems.
This is not a good technical solution as an enterprise solution should never cap data, but work with ways to ensure high quality performance using web technologies to cache and limit how much data is loaded on load and on user action. All data should always be available on demand.
If you have any questions
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