Windows has an "8k limit" on the horizontal pixels. Not sure if this means 8,000 or 8,192, but your scenario is well past both. You can do 5 panels in portrait mode - but not landscape.
Disclaimer: I only know about the nVidia side of the fence as far as hardware capabilities. My post is based on that knowledge.
It's not really Windows that sets the limit, it's the video card hardware. On the nVidia side of things 8000, 9000, 200-series all are 8192x8192 maximum. You can find the theoretical maximum that a GPU can handle by downloading the DirectX SDK and using the DXCapsViewer app to check the properties of the hardware you're using. It's not a Windows limitation and it doesn't have to be 8192 maximum either. It's whatever they design the video card to handle. :wink:
As far as 8400 pixel width, that's not going to happen with the hardware that is currently on the market, because no current gaming GPU has been designed to use more than 8192 pixel width.
As far as I know, 5000-series cards can not bridge GPUs/cards to do EyeFinity. My understanding is if you have 2 EyeFinity capable cards, that you only use the port outs on 1 of the cards, not both. EyeFinity and CrossFire I understand is working, but the extra cards are only helping process the graphics, not give extra ports out. (If I'm wrong about this, someone speak up and correct me, as I don't have an EyeFinity setup to definitively confirm this) If you want to run more than three displays, it'll require the EyeFinity6 card, with 6 mini-dp ports.
If you're thinking 5x displays in landscape orientation the largest you can do is 5x 1600x900 or 1600x1200 for a resolution of 8000x900 or 8000x1200. 1680x1050 pushes over the 8192 maximum on current cards. If you want to keep with 1680x1050 panels you could do 5x 1680x1050 in portrait orientation for a resolution of 5250x1680.
Keep in mind that 5 displays in landscape mode is going to require some forethought on the desk you use. It probably would only work on a L or corner desk setup because of the desktop layout required for the displays. I've used a 5x display setup previously using CRTs (softth + th2g analog) and it's pretty impractical for everyday use. Just because you can do something doesn't always mean that you should, or that it's actually practical. I still think 5x 24" LCDs in portrait mode would be great. I'd love to have a 6000x1920 setup. :wink: