The ASUS A88X-PRO comes with two PCI Express 3.0 x16 slots, one PCI Express 2.0 x16 (that always works at x4 speed), two PCI Express 2.0 x1, and two PCI slots. The PCI Express 3.0 x16 slots share bandwidth, which means that if the second one is unused, the first one works at x16 speed, but if both are populated, they will both work at x8.
You also need to keep in mind that PCI Express 3.0 is only supported if an FM2+ processor is installed. With an FM2 CPU, the slots only support PCI Express 2.0.
The portrayed motherboard supports CrossFireX with up to three video cards.