You could easily get 4 and possibly 6 pork butt roasts in it with just an Adjustable Rig R&B from the Ceramic Grill Store, but it would definitely be tight with 6.
This one.
I have that and also the
Extender which makes cooking on both levels a bit easier.
I have 2 levels to cook on plus the lower position for a drip pan (which you definitely want for almost all smoking cooks on a BGE). I have the oval stone that comes the adjustable rig, so that takes up the lowest level if indirect cooking leaving you with two levels to cook on (I put the oval stone on and the drip pan goes directly on the oval stone, taking up the lowest level). If you used the BGE convector and you could possibly use all 3 levels for cooking, you could put a whole bunch of butt in a large BGE. You might could put a drip pan below the lower 3rd grid level directly on the BGE convector stone, but I've not tried that, so can't confirm that. You gotta have something and definitely don't want all that grease running down into your charcoal or ceramic. I've seen pics of people putting each butt in a disposable aluminum pan and stuffing them in too.
Lots of different ways to rig it up and that Adjustable Rig is the most versatile setup I've seen. Since I've bought mine BGE is offering their own version of it and it has some merits, but it seems the CGS model still wins out. Both are quality builds out of heavy stainless and will last for many years.
ETA: I'm pretty sure you could do a whole chuck. I've done whole packer trim briskets in mine many times. Sometimes I have to fold part of the flat under if they are really large, but it doesn't hurt anything at all. IIRC, the cooking grid is 18" or 19" in diameter.