It is important to keep in mind that, at AM4 platform, only the PCI Express lanes controlled by the CPU (24 on total, 16 for the video card(s), four for an M.2 slot and four for the communication with the chipset) are PCI Express 3.0; the PCI Express lanes controlled by the chipset are use the 2.0 standard.
The ASRock X370 Taichi comes with two PCI Express 3.0 x16 slots, one PCI Express 2.0 x16 slot (working at x4 speed maximum) and two PCI Express 2.0 x1 slots.
The PCI Express 3.0 x16 slots share the same 16 lanes, so they can work at x16/x0 (if the second one is unused) or x8/x8 configurations.
There are also two M.2 slots, the first one supporting both PCI Express 3.0 x4 and SATA-600 SSDs, and the second one supporting only PCI Express 2.0 x4 devices.
The PCI Express 2.0 x16 slot shares its lanes with the second M.2 slot. So, if it is in use, the second M.2 slot will be disabled.
The motherboard supports CrossFire and SLI with up to two video cards.
The first two 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