Personally I don't think that anyone needs that many screens, but if it helps them be a better trader, why not... The only thing with THIS specific card is that it is very expensive, gets very hot and makes a lot of noise. If you really 'need' that many screens, there are better solutions.
How many slots do you want? You're not limited by your mobo. http://www.cyclone.com/products/expansion_systems/600-2707.php
This gadget is $1800. Price higher than most high-end PCs itself. It depends on what one needs. Perhaps the display can be split over 2 computers.