This is from the Apple website
Dual display and video mirroring: Simultaneously supports full native resolution on the built-in display and up to 2560 by 1600 pixels on an external display, both at millions of colors
So it depends what you are trying to achieve. You could have the laptop display showing one image and a single external display showing another (dual display) or you can mirror the laptop display on an external monitor.
If you connect more than one external display it will only be able to show the same as all other external displays as this is all the graphics card can drive.
For multiple monitor support you would need a Mac Pro (tower) with the following video options
ATI Radeon HD 5770 with 1GB of GDDR5 memory, PCI Express 2.0, two Mini DisplayPort outputs, and one dual-link DVI port
ATI Radeon HD 5870 with 1GB of GDDR5 memory, PCI Express 2.0, two Mini DisplayPort outputs, and one dual-link DVI port
Option to install two ATI Radeon HD 5770 cards
Support for up to six displays
Support for digital resolutions up to 2560 by 1600 pixels
Support for analog resolutions up to 2048 by 1536 pixels
Nikkor AF-S DX; 18-105 f/3.5-5.6G VR, 55-300 f/4.5-5.6G VR, 35mm f/1.8G
Speedlight SB-700 http://keystrokesukimages.com