I want to know that how may folders we can create in document library in SharePoint 2013.
In msdn article they said that 30,000,000 documents per document library, but what about folders per document library?
You can create very large document libraries by nesting folders, or using standard views and site hierarchy. This value may vary depending on how documents and folders are organized, and by the type and size of documents stored.
But there is no such limitation on folder creation in SharePoint 2013 Document library. But yes, your document library can contains upto 30M items(Documents+folders).
Documents and folders all are treated as items.
So the limit mentioned consist of all including folders as well.
To test it, write a query which will return total count. and another query where item is not folder. It should show the difference. Which in terms say folders are also treated as items.
PS: Folders also affects threshold limit.