IIRC in the old days the DX was bound to Windows OS versions largely, with each release being iterative improvements over the previous. But DX1 through 11 are largely the same render path, at least there's significantly more similiarity between 9 through 11. In the end MS stopped after DX11, to keep that as a high level API. And DX12 as their low level API. If there were a DX13, I would agree with Lurkmass, it would be a different rendering path than we have today. I assume at this point in time, they will just add more to DX12 and DX11 unless there is something out there that would suggest a completely new render path would make more sense in the future.
There's not really any rhyme or reason behind the numbering system here - it's really just an extension of marketing, but it seems to have settled to high level (DX11) and low level (DX12)