There are three screen versions on the W520 which are HD (1366x768), HD+ (1600x900) and FHD (1920x1080). The graphics card can handle even more. Obviously it also depends on what kind of things you are displaying. For example, the higher the resolution the more performance will drop on demanding 3D games and applications.
|The Lenovo ThinkPad W520 is a "mobile workstation" and has several outputs that allow it to connect to multiple monitors. The system has a VGA and DisplayPort which allow up to two monitors to be connected at the same time.|
According to the Data Sheet you can also use the ThinkPad Mini Dock Plus Series 3 170W to achieve three external monitors at the same time. They call it quad displays (3 external monitors, and W520's screen)