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

USB PCIe card in x16 slot? pcie slot x16 It's a slot of x16 size but electrically x4 This pcie 4 ssd in pcie 3 slot It is used for GPUs mostly as well as SSD cards PCI Express x16 Slot – This is the largest and used for high-performance GPUs How Does PCIe Slot Work?

pcie cem slot PCIe compatibility and performance generates a LOT of confusion In Explaining PCIe Slots