Two 24" is probably max what you can fit.. the desk lacks width for more unless you wanna go vertical-horizontal-vertical setup.
Finish on this desk is superb, no colour patches at all, nice shade of black.
Extra shelf was invented by me, it wasn't part of the desk
Its a combination of wall mounted shell and legs, check here for more info:
http://www.ikea.com/gb/en/catalog/products/60103632
http://www.ikea.com/gb/en/catalog/products/00054564
Legs come in different heights, depends on your needs [three different types iirc]
I really like the idea as you can have lots of free space under my screen(s)
Also, what I use and its really the most important thing in this setup - cable management tray:
http://www.ikea.com/gb/en/catalog/products/50035115