Apple - How many monitors can the 13" retina MacBook Pro output to?

You can drive 3 including the internal on the 13". However, you cannot run 3 external displays even if not using the internal.

source: I have one and have tested this.

According to Apple (http://support.apple.com/kb/ht5219):

4 MacBook Pro (Retina, Mid 2012), MacBook Pro (Retina, 13-inch, Late 2012), and Mac Mini (Late 2012) computers can use an HDMI-compatible device on it's HDMI port while using one Thunderbolt display, or they can use two Thunderbolt displays.

The new Retina 13 is able to daisychain and drive two thunderbolt cinema displays, as explained in this very nice article:

Since the 13" MacBook Pro uses the same Intel HD Graphics 4000, DSL3510L Thunderbolt controller, and single physical Thunderbolt port as the new MacBook Air, it should be able to push a total of three monitors using daisy-chained Thunderbolt Displays.

source: Ars Technica

However, it's not clear if they can at the same time drive an extra display though the HDMI port (for a total of four including the laptop's display) like the 15 inches model can do. The 15 inches can drive 3 external and the retina display, but it has a very powerful discrete video card.

Considering that there are a lot of pixels in that configuration, I highly doubt that the integrated Intel 4000 can do it. It's a very limited video card with shared memory. But again, we'll need to find someone who can try it :)