Locate a motherboard you like (which will be new and fairly high end), look up the specs on the manufacturer's website. Can't really trust online listings at Amazon or other e-tailers, people can pretty much write what they want. You can probably trust the ones that use the official marketing images though.
Considering there are no mainstream PCIe 5.0 graphics cards, not that important. Unless you wanted to use the x16 slot for something else, in which case you would probably be after a motherboard that supports bifurcation. By the time PCIe 5.0 graphics cards are standard, you might want to consider upgrading to something current.