I have found my own answer... From a review of the MSI X99S Gaming 9 AC Mobo
http://www.guru3d.com/articles-pages/msi-x99s-gaming-9-ac-motherboard-review,5.html
PCIe Lanes
The one thing that people grumble about the most is the relatively small number of available PCIe lanes (16) for graphics cards. If you are using a setup with two or more graphics cards (SLI/Crossfire), the PCIe bandwidth is limited at x8:x8. Here again the performance difference is trivial as running today's fastest cards barely utilizes all that bandwidth, but two x16 and then room for another x8 obviously is much better.
The eight core Core i7 5960X Haswell-E has 40 lanes (PCIe Gen 3.0) available.
The six core Core i7 5930K Haswell-E has 40 lanes (PCIe Gen 3.0) available.
The six core Core i7 5820K Haswell-E has 28 lanes (PCIe Gen 3.0) available.
You can split them up in a variety of combinations, two x16 links with one x8 link, one x16 link and three x8 links, or one x16 link, two x8 links, and two x4 links. So some examples, Intel Core i7-5960X and Intel Core i7-5930K Haswell-E would support multiple graphics card configuration with the first two cards running at x16 speed and say a third one running at x8 speed. Meanwhile a Core i7-5820K would only support 1 x16 + 1 x8 + 1 x4 multiple graphics card configuration. Again, all are Gen 3.0 so really (read plenty) that is a lot of bandwith.