The ASRock Z270 Extreme4 comes with three PCI Express 3.0 x16 slots and three PCI Express 2.0 x1 slots.
There are also three M.2 slots: one up to 2280, one up to 22110 (both supporting both PCI Express 3.0 x4 and SATA-600), and one 2230 for Wi-Fi modules.
The two first PCI Express x16 slots work on x16/x0 and x8/x8 configurations, which means the first slot work at x16 if the second one is unused, but it it is occupied, they will both work at x8.
The third PCI Express 3.0 x16 slot works always at x4 speed.
The motherboard supports CrossFire with up to three and SLI with two video cards.
The two first PCI Express x16 slots are covered by stainless steel shields, that work as a shield against interference, and also improving mechanical strength of each slot.
Figure 2: slots
Figure 3: second M.2 slot
Figure 4: third M.2 slot, for Wi-Fi cards