When you are creating projects under one collection on TFS 2013 there is still one database for each team project collection (name should be Tfs_Collection). For reference: http://msdn.microsoft.com/en-us/library/ms400720.aspx.
Has it changed by supporting GIT as second version control? What if I have one collection with projects based on both vc's - GIT and TFVC? Still one Tfs_Collection database?