On this page the maximum number is mentioned as 7. http://technet.microsoft.com/en-us/library/cc262787(office.15).aspx
When you try to add more levels manually it seems to work with no problems.
I have also tried to test the limit using CSOM from C# console application. Stopped when I reached 2900+ levels. So this should mean that you can create more levels but then some performance issues could occur.
There are couple of notes on the TechNet article mentioned above:
Boundaries: Static limits that cannot be exceeded by design
Thresholds: Configurable limits that can be exceeded to accommodate specific requirements
Supported limits: Configurable limits that have been set by default to a tested value
Some values in this article are based on test results from SharePoint 2010 Products and may not represent the final values for SharePoint Server 2013. This article will be updated with appropriate values as SharePoint Server 2013 test data becomes available.