Socket LGA2011-v3 processors have a maximum of 40 or 28 PCI Express 3.0 lanes for video cards, depending on the model. This allows a very high-end configuration for the PCI Express x16 slots when a CPU with 40 lanes is installed, but the exact configuration used will depend on the model of the CPU that will be installed.
The ASRock X99 Extreme6/3.1 comes with three PCI Express 3.0 x16 slots, working at x16/x16/x8 if a 40-lane processor is used, and at x16/x8/x4 if a CPU with 28 lanes is installed. There are also two PCI Express 2.0 x1 slots, and a mini-PCI Express slot.
The motherboard supports SLI and CrossFire arrays with up to three video cards.
There is also an M.2 slot, which is compatible with PCI Express 3.0 x4 and SATA M.2 modules. If a PCI Express x4 M.2 module is installed, the third PCI Express 3.0 x16 will be disabled.