Running one more display when MacBook is in clamshell mode I was wondering if anyone has tried running one more screen off their MacBooks by closing the lid of the MacBook. This is a common practice of many Windows laptops that support only 2 screens but have multiple video outputs. Since this is a hardware limitation of the number of video out ports and the number of screens that the video card in the laptop supports I would assume that any these combinations should work, if not in OS X but in Windows under Boot Camp at least.

Retina MacBook Pro: Supports two screens through any combination of HDMI and Thunderbolt/DisplayPort; lid closed: three screens though the same any combination of the video out ports.

MacBook/MacBook Pro 13"/MacBook Air/MacBook Pro 2012 (non-retina): Supports only one external screen with the internal one on, but when lid closed should be able to connect one more Thunderbolt display.

MacBook Pro (Before 2012 edition with Thunderbolt and AMD graphics): Two Thunderbolt screens connected with the lid open; lid closed, three Thunderbolt screens. This possibly may not work because of bandwidth limitation of Thunderbolt, not that the video card can't handle it.

Clarification: The configurations I list are what should happen if my hypothesis is true. I am asking for people to confirm this by using thir own MacBooks since I do not own this hardware to test.
Here is some documentation.

http://support.apple.com/kb/HT5219?viewlocale=en_US&amp;locale=en_US#dispnum

Specifically this, noting the footnotes. I've done some editing for readability:

How many Apple Thunderbolt Displays can I use with my Mac in OS X?

The table below indicates how many Thunderbolt displays can be connected to Thunderbolt-capable Mac computers in OS X.

Computer    Maximum number of connected displays
Thunderbolt-capable Macs with only Intel HD Graphics 3000 integrated graphics can support one connected Apple Thunderbolt Display (27-inch):


  MacBook Air (Mid 2011),
  MacBook Pro (13-inch, Early 2011) and (13-inch, Late 2011),
  Mac mini (Mid 2011), 2.3 GHz,
  Mac mini with Lion Server (Mid 2011)


Thunderbolt-capable Macs that can support up to two connected Apple Thunderbolt Displays.


  MacBook Pro (Retina, Mid 2012),
  MacBook Pro (13-inch, Mid 2012),
  MacBook Pro (15-inch and 17-inch, Early 2011) and later,
  MacBook Air (Mid 2012),
  iMac (Mid 2011),
  Mac mini (Mid 2011), 2.5 GHz,


Note: The F8 key does not work when using Windows with a USB keyboard connected to an Apple Thunderbolt Display (27-inch).

1 You can connect a second Apple Thunderbolt Display (27-inch) to a MacBook Pro (13-inch, Early 2011) and (13-inch, Late 2011), but the built-in display on the MacBook Pro will go dark. This is expected behavior.

2 iMac (27-inch, Mid 2011) with two Thunderbolt ports supports a total of two Thunderbolt displays regardless of which Thunderbolt port each display is connected to.

3 Mac mini with AMD graphics can support an HDMI compatible device on its HDMI port when using two Thunderbolt displays.

4 MacBook Pro (Retina, Mid 2012) can support an HDMI-compatible device on its HDMI port while also using two Thunderbolt displays.

And here's an OWC article with an image showing a MacBook Pro with Retina that has three attached displays plus the built-in.

http://blog.macsales.com/14241-macbook-pro-15-with-retina-display-can-run-3-external-displays