Navigation's "Horizontal with tabs" template displays up to 2 levels.
Attached is a crude version of the template that can support up to 5 levels. However, if you have such deep nested navigation, I would recommend using a different type of navigation template if possible.
Please let me know if I can be of further help.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items