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

Solution 1:

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.

Solution 2:

No, the Macbook Pro Retina can support:

Dual display and video mirroring: Simultaneously supports full native resolution on the built-in display and up to 2560 by 1600 pixels on up to two external displays, at millions of colors

(emphasis mine)

Source: http://www.apple.com/macbook-pro/specs/

update: It has been dully noted that using the included HDMI connector one can run a third display using a max resolution of 1920 x 1200. It is important, however, to note that one cannot daisy chain more than two thunderbolt displays (at least according to Apple so far).