Other World Computing posted this picture on their blog showing a MacBook Pro with Retina Display hooked up to a pair of iMacs serving as Thunderbolt displays and a third monitor via HDMI. This setup powers four screens with a total of 15,680,000 pixels.
The writer of the post, OWC Mike, seemed impressed with the performance of the MacBook Pro:
Apple officially supports hooking three monitors into the Retina MacBook Pro, noting in the Thunderbolt ports FAQ in its Support Knowledge Base:Moving images and media didn't create any lag and we were able to play video on all four displays simultaneously.
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.MacBook Pro (Retina, Mid 2012) can support an HDMI-compatible device on its HDMI port while also using two Thunderbolt displays.
Article Link: Retina MacBook Pro Can Run Three External Displays Simultaneously