Tested graphics card:
ASUS Radeon HD7970-DC2T-3GD5 => 4 screens without problems (up to 6 can normally but I have not the necessity).
Sapphire HD 7970 Vapor X GHz Edition 3GB => 3 max screens (restriction of the graphics card).
Sapphire Radeon HD 6950 => 3 screens.
Screen test:
ProLite E2607WSD-B1
ASUS VW266H
Samsung P2770HD
Wacom Cintiq 12WX
Be careful not to exceed a resolution of 1920 * 1200 screen otherwise you will need the adapter to over 100.