Apple - How many external monitors does the iMac 27" with 5k display support?

The Late 2015 iMac 5K with AMD 395X can support two external 4K (UHD) displays (at the full 3840x2160 res) with the builtin iMac display set to 5120x2880 FYI. This is the configuration that I am currently running (with 10.11.2), using two LG 27MU67 displays attached via DisplayPort-to-miniDP cables on each of the iMac's thunderbolt ports.

From Everymac…

In addition to the internal display, this model can support an external display up to 3840x2160 via Thunderbolt 2 (4K UltraHD). Third-parties have discovered that it also can support two simultaneous 27-Inch Thunderbolt displays at 2560x1140 each in lieu of a single UltraHD display.

I'm tempted to think that 2560x1140 is a typo & ought to be 2560x1440

At least 2, and likely no more.

The Retina 5K iMac can support an external 4K (UHD) (3840x2160 @60Hz) monitor AND a second external monitor up to at least 1920x1200 (60Hz) at the same time.

This is the setup I am currently using. No special tricks needed. Machine: Core i5 3.5 GHz, Radeon R9 M295X 4GB VRAM graphics card (BTO upgrade), 16 GB RAM, Yosemite 10.10.5.

I have the internal display set to scaled ("Looks like 2880 x 1620") and the 4K display set to 3840x2160 in Display Preferences. These settings likely result in a reduction in performance vs. using the "Best for Retina" settings.

User interface animations (e.g., Mission Control) can be a little slower with these external displays running, but frankly it's not that snappy even without the external displays, if I have a lot of windows open. Hopefully El Capitan should fix this. I don't do anything graphics-intensive, so can't comment on gaming/pro apps performance.

Things I've done to improve UI performance:

• System Preferences > Accessibility > Check "Reduce transparency"

• Deactivate the Mission Control Animation with the following command in Terminal:

  defaults write com.apple.dock expose-animation-duration -int 0; killall Dock
  

  • To set Mission Control animations back to defaults:

    defaults delete com.apple.dock expose-animation-duration; killall Dock
    

• Used an app such as QuickRes to turn off the OSX Retina/HiDPI mode on the 4K display. Since I'm running it at native resolution, Retina/HiDPI pixel-doubled drawing puts pointless effort on the GPU (i.e., it doesn't serve to make anything appear crisper on screen).

FWIW, this document from Apple says that the Retina iMac can support two Thunderbolt displays (2560x1440). It also says that each Thunderbolt port can only support one display of any kind, and so that would limit the iMac to two displays total. It also suggests that the Retina iMac can support a 4K display (using MST) AND a Thunderbolt display at the same time (see footnote 5 in the table describing number of supported Thunderbolt Displays). That configuration is likely the maximum number of pixels the machine can put out across two displays (or possibly 4K & 2560x1600).

If you want to have more than 2 monitors, you can always use USB adapters utilizing DisplayLink's chipsets. I've used these to good effect on my Retina iMac.