[Build Help] 12 displays 2 Video Cards

Hello! I am looking at hardware to build a PC(s) for 20 displays. All 1920x1080, monitors on a wall. I am thinking I can build two PC's with 2 Visiontek 7870 (so 12 mini-displayports each). Do I need to find a motherboard that will have two dedicated PCIE 16x ports or will it be okay if one goes down to 8x? This computer will only be running webpages on each screen in full screen mode showing social media and web stats so nothing too crazy. Thanks!