A single 12 oz will be plenty. I wouldn't imagine they'd sell it by more than that, but if it's only 6ers I'm cool with that too. I've usually seen folks having what looked like bombers, which would be fine too. I usually like to trade 6ers though, and would be more than happy to fill out the rest with other stuff.
I'm not sure what's available my way that's not available your way, and I'm sure there's a lot of overlap. I can easily get most anything from Terrapin, Troegs, Weyerbacher, Victory, Heavy Seas, Flying Dog, Duclaw, Sixpoint, Brooklyn. Can get just about anything from Dogfish Head. A few DC area locals that are really good (DC Brau, Port City). A number of breweries elsewhere in VA (Williamsburg Alewerks, Star Hill, Blue Mountain, and Devils Backbone, who owned at GABF last year). Highland and Duck-Rabbit from NC. Can get Allagash and Maine Beer Co (although limited selections). I'm sure there's a lot more I'm not thinking of at the moment. Basically most anything east coast (no Cigar City or Alchemist though). Anything in particular you're looking for?