GeForce based graphics cards are supported in single, 2-way, 3-way, and 4-way multi-GPU configurations, depending on the specific product. This support encompasses both SLI graphics rendering and multi-GPU general purpose computing.
For more information on the Geforce family of consumer graphics products, click here: GeForce Product Family
For system configurations with more than 4 GPUs, NVIDIA recommends Quadro or Tesla solutions.
For more information on Quadro professional graphics solutions, click here: Quadro Product Family
For more information on Tesla computing solutions, click here: Tesla Product Family