How many graphics cards does minecraft use at max?

Minecraft is not really graphics hungry game anything more than single GTX 600 700 and future 800 series would be total overkill.
I just find out Minecraft does not support nor SLI nor Crossfire.

As many as you can put in I'm pretty sure, most games take advantage of sli and crossfire support, not sure why you would need two gpus to play mc though

What Minecraft is rough on is RAM and Processor, exponentially if mods get involved.

It dosen't really matter about the Graphics card what matters is the CPU.