How many external monitors can my computer support?
The total monitors that your computer can support including builtin monitor is based on the GPU, monitors that you use, bandwidth for displayport signal and resolution.
Please check with the manufacturer for more information on the external monitor support. For example, if you have MacBook Pro Retina, 13-inch, Late 2012, you can visit its website
http://support.apple.com/kb/SP658 and you should look into the "Graphics and Video Support" section for more information.