It seems that SLI is becoming synonymous with two GPUs only, and NVIDIA won’t be supporting more than 2-way setups in the near future. That’s not necessarily bad news, considering that scaling of performance is not linear after two, however those that wish to have the absolute ultimate in performance at any cost won’t be able to anymore.
NVIDIA will only officially support 2-way SLI going forward
Originally NVIDIA planned on supporting SLI with more than two cards in a limited fashion. The plan was that you’d be able to sign-up for an “Enthusiast Key” that would be used to allow three or more cards to be connected together logically. That plan never materialized and now a statement by NVIDIA to PCper says quite plainly that they’re “focusing efforts on 2-way SLI only and will continue to include 2-way SLI profiles in Game Ready Drivers.” They even tested and found that only two GPUs were ever capable of being joined together in harmony by the driver. All four installed were detected by Windows, of course.
Oddly, for those extreme benchmarking enthusiasts, NVIDIA will still be creating and updating SLI profiles for more than two cards for popular benchmarks such as 3DMark, Unigine and Catzilla, though the support for more than two cards. In DirectX 11 and OpenGL, at least. OpenCL will, naturally, take advantage of as many GPUs as there are in the system so long as it’s written to do so.
In DirectX 12 and Vulkan there exists Explicit Multi-Adapter mode which, when utilized by a game, can access any and all GPUs present in the system regardless of multi-GPU profiles in the drivers. That technology even allows AMD, NVIDIA and Intel to combine their GPUs together to increase performance. At the moment only Ashes of the Singularity takes good advantage of that, however. More games in the future could easily start using that, or at least implementing it at a low level. Stardock also has plans to implement their own platform agnostic tool that allows multiple GPUs of differing manufacturers to be used together in a similar manner as Explicit Multi-Adapter but with any low-level API.