recentf-arrange-by-rules-min-items

recentf-arrange-by-rules-min-items is a variable defined in `recentf.el'.
Its value is

0

Documentation:
Minimum number of items in a `recentf-arrange-by-rule' sub-menu.
If the number of items in a sub-menu is less than this value the
corresponding sub-menu items are displayed in the main recent files
menu or in the `recentf-arrange-by-rule-others' sub-menu if
defined.

You can customize this variable.