You can put 6 gcards in your motherboard - but you'll need slots and the right chipset to distribute the load, plus a dual quad core cpu.... :lol: Just joking.
As far as i know, there is no such option on normal motherboards. The 975 mobo from Intel, does have 3 slots, but alas only two will work in dual mode with Crossfire. It was suppose to do SLi aswell, but some sour grapes exists because NV don't feel comfortable with the idea of a mobo who supports it's own product, and the oppositions in dual mode. Ofcourse, it's more political than this.
Was your question a general question, or do you wish to actually try and rig 6 monitors?