The MSI B350 TOMAHAWK comes with one PCI Express 3.0 x16 slot, one PCI Express 2.0 x16 slot (working at x4 maximum speed), two PCI Express 2.0 x1 slots, and two PCI slots.
The PCI Express 2.0 x16 slot that works at x4 speed shares its lanes with the PCI Express 2.0 x1 slots. So, if any of them is in use, this slot will work at x2 speed.
The motherboard supports CrossFire with up to two video cards, but it does not support SLI.
There is also one M.2 slot, supporting both PCI Express 3.0 x4 and SATA-600 SSDs.
The PCI Express x16 slot is covered by a stainless steel shield, that works as a shield against interference, and also improves the mechanical strength of the slot.
Figure 2: Slots