It doesn't directly answers your question, still good to keep in mind:
It could be the maximum number of:
-32,767 databases per Server;
-2 bln objects (tables) per database;
-1024 columns per table
-8060 bytes per row
I didn't find anything else related to your question, so I could say it may be limited, based on the above, to your business need/common sense and physical space.