There's more than 6! It's still ongoing at the moment, I'm guessing there will be maybe 10 volumes. Not long at all.
Yes, and there are spin-offs : Nightmare of Nunnally and Suzaku of the Counterattack.
I heard the counterattack one sucked, I've read 3 volumes of the Nunnaly one, waiting till I can pull some more cash in a buy the rest of it, it's only 5 volumes. I think the counterattack one was like 2 volumes, but I'm not sure.