With the new Mining boards, you can handle on the moment actually 15 GPU on Linux Ubuntu.

Does Linux support multiple GPU?

In Linux, with two GPUs SLI and Multi-GPU can both operate in one of three modes: Alternate Frame Rendering (AFR), Split Frame Rendering (SFR), and Antialiasing (AA). When AFR mode is active, one GPU draws the next frame while the other one works on the frame after that.

Can you run 4 GPUs?

Read under “Expansion Options”. So, if you want to use quad GPU setup, you’ll need a server CPU that has support for 4x GPUs. Do note that even the latest consumer grade CPUs, e.g i7-8700K, doesn’t support more than 3x GPUs.