The first two slots (from the processor connector) with the PCI Express x16 form factor are implemented using 16 PCIe 3.0 lines of the Skylake-S processor, which, with the use of multiplexers/demultiplexers (ASM1480) , are grouped into either one PCI Express 3.0 x16 port or two PCI Express 3.0 x8 ports. That is, if only one slot with the PCI Express 3.0 x16 form factor is used (the one closest to the processor socket), it will run at x16 speed, and if both slots are used simultaneously, they will run at x8 speed.
Another slot with the PCI Express x16 form factor is implemented on the basis of four PCI Express 3.0 lines of the Intel z170 chipset.
In fact, it is a PCI Express 3.0 x4 slot, but in the PCI Express x16 form factor.
Note that the Asus Z170 Pro Gaming Board supports Nvidia SLI and AMD CrossFireX technologies and allows installation:
* of two Nvidia video cards
* and up to three AMD video cards.