Briquettes or lump?
Briquettes; you'll need at least 12 lbs to be on the safe side.
Lump; I'd go the same, as closing down at the end will snuff out the fire once the meat is off the pit.
On a 22" grate you could get 4 7 - 8 lb butts, but if you're cooking a packer brisket, and 2 large butts it's going to be a little tight, find yourself a can, (beans, tomato sauce etc), take off the top and bottom and burn off the paper, then do the brisket drape over the top of the can, this allows it to fit while cooking during the first few hours, then as it shrinks you can take the can out and still have room.
"There is no such thing as a little garlic." A.Baer