On a B550 board you have 20x PCie lanes that are 4x out of 28 lanes. 4 lanes for other PCIe devices and 4x lanes to Chipset. The problem is the chipset only has Gen 3 lanes between it and the CPU. 16XGen 4 lanes are dedicated to PCIe slot 1, and 4x Gen 4 lanes direct to the CPU from the first M.