How many external displays can I power with the 15" retina MacBook Pro?
The Other World Computing Blog shows that the new MacBook Pro 15" with Retina Display Can Run 3 External Displays:




  image source: http://blog.macsales.com/


These resolutions were used:


Retina on laptop @ “best for Retina”
iMac used as a display @ 2560 x 1440 via Thunderbolt
iMac used as a display @ 2560 x 1440 via Thunderbolt/DisplayPort
LG monitor @ 1920 x 1200  via HDMI


Quote from the MacRumors news article on that:


  This makes the Retina MacBook Pro the first Mac -- other than a tower-based workstation like the Mac Pro -- to natively power four displays simultaneously.