When a page is added to the favorites by an user, Confluence internally assign a label to the page.

In Confluence, there is a limit of 500 labels per item. This is hard-coded as a constant in the class com.atlassian.confluence.util.LabelUtil:

    /**
     * The maximum number of labels allowed on a page or space.
     */
    public static final int MAX_ALLOWED_LABELS = 500;

This limit is insufficient for instances with thousands of users. There should be no limit on how many users can favorite a page.