I want to update my 27'' monitor x 6, so RX 580 hold it? Use: Trades applications and multimedia (Videos,Photos,Rec).
It will support as many monitors as you can plug into it. Just be sure to get one with 6 video outputs (or get ready to run a displayport hub) Gonna have to use several different adapters (hdmi, dvi, DP) unless you use displayport hub. The easiest way to do it would be to get a workstation card that comes with 6 DP ports, but yes, you can do it.
If it's only for video output and not for gaming or heavy aplications, you could buy a cheaper AMD GPU with 6 hub ( i remember i saw a 7750 for example xd)
For the rest of us, if you have older spare cards laying around from the same brand, you can stick those in the PC, too, if you have extra x16 slots (or open-ended slots with less PCI-express connectivity), without running crossfire or SLI. These will power extra screens. I keep a box of 8xxx series OEM cards / GF 710 cards around here for that sole purpose as I do content creation here. Helps when you have CAD stuff etc that needs extra screens, but don't want to slow down the primary card or make it ramp up the fans (holding higher VRAM clocks to feed all the different screens) all the time. I wouldn't mix NVidia / AMD brands solely for the fact you'd have two different brands of drivers on the system, and those two being competitors don't always have a history of playing nice (though it can play nice about as much as it COULD cause a problem). Intel plus either brand doesn't tend to cause issues, as due to laptops with add-in video boards for 3D have honed this dual-brand compatibility better.