Running one more display when MacBook is in clamshell mode
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