Hello Thomas Hansen,
The default templates for RadMenuItem do not include ScrollViewer. This is why when you have a lot of items some of them are hidden. To workaround this you can edit the templates and add ScrollViewer.
Please note that RadMenuItem have four templates. You should edit only those that have Popup control in them (submenu items are placed there) - TopLevelHeaderTemplateKey
Let us know if you need further assistance.
the Telerik team