How many displays on MacBook Pro 2011
Yes, you can, since your computer has a Thunderbolt port (http://support.apple.com/kb/sp620). According to Apple, you can daisy-chain additional monitors using the Thunderbolt port:


  To make a desktop workstation out of your MacBook Pro, just daisy-chain additional Thunderbolt devices, including high-performance storage and video and audio capture devices, through the display. (http://www.apple.com/displays/, section "Expand the capabilities of your Mac")


Two monitors connected to one MacbookPro: