How many monitors can the new retina MacBook Pro handle and at which pixels resolutions?
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.