What does PCIe-x16 really mean and how many do I need?

Are UAD-2 cards supported on PCIe x4 or x16 slots? pcie slot x16 Graphics Cards · 5770 supports PCI Express up to x16 · Radeon RX 550 supports PCI Express up to x16 · PCI Express x16 is PCI Express pcie 20 x1 slot It's a slot of x16 size but electrically x4 This

pcie 4 slot Have you ever thought if you can plug PCI Express cards into x16 lane slot? In this video