Doesn't work - I've been working on it for a week and half. The best I can do is 5 and even then, it's not completely stable. GPU 1 (if you start with GPU 0) on all 3 machines get a error code 43 from time to time. It's easy enough to fix, but it's causing stability issues. I can only get the machines to run for 3-4 hours at a time before GPU 1 hangs in open cl. At first, I thought it might have something to do with the video card, but whatever it is - it's exactly the same issue across all 3 machines (MSI Z170A Gaming Pro Carbon - MSI RX 580 - Windows 10 - AMD driver 17.4.4). I've tried several different setups, bios changes, changing intensity levels, using -old 1, etc. I can't seem to fix it.
My best guess is - AMD driver. I came across a post from AMD that says their drivers only support 4 cards and no more. Whatever they did, you cannot get 6+ cards under Windows and 5 seems to be unstable. 4 works fine.
Until AMD comes out with a new driver or someone figures out how to modify the driver, we are screwed and have to look to other options such as unbuntu. I've heard rumors that some have gotten 6 cards working with unbuntu.
Good luck! If you find anything let us know