AJA makes a couple of Thunderbolt 3 external SDI capture/output devices with 4x bidirectional SDI I/Os. If you want more than that you need multiple devices or

AJA makes a couple of Thunderbolt 3 external SDI capture/output devices with 4x bidirectional SDI I/Os. If you want more than that you need multiple devices or a PCIe card. AJA and Blackmagic both make 8x SDI cards, as do a couple other vendors.