The retina MacBook Pros can do up to 3 external monitors. They have 2 thunderbolt ports (each capable of running a DisplayPort, DVI or TB monitor) and an HDMI port.
The Airs can do 2 TB monitors, daisy chained. You can also do a TB monitor and a DVI/Display Port monitor in a daisy chain, provided there is another thunderbolt device in between (this is due to the TB/Display Port implementation).
You can, as has been previously mentioned, also use a USB video adapter. As of 10.9, you can also use an AppleTV to AirPlay wirelessly to a monitor/TV.