A 400 Gb/s link carries about 50 GB/s in each direction (400 divided by 8) before Ethernet overhead. The PCIe link has to offer at least that much per direction.
As a rough guide, Gen5 runs at 32 GT/s per lane and gives about 3.9 GB/s per lane per direction after encoding, so an x16 Gen5 link offers roughly 63 GB/s and an x8 link about 31.5 GB/s. An x16 Gen5 slot therefore has room for one 400G port; x8 does not. At Gen6, the headline rate doubles, so an x8 Gen6 link reaches about 64 GB/s.
These are raw link figures. Protocol overhead, packet size and the host platform reduce what software sees, so leave margin. Also check that the card, the slot and the platform all support the generation you are counting on, since the link trains down to the lowest common speed.
