That will be a nice Christmas!
I would guess that if you were organized about it you could put 4 per shelf without any problem, so 16 total in the 40". I once had 8 butts in mine and everything fit well and they're about twice the width of a fatty give or take.
I would be sure to preheat first (and I would preheat higher than what you are going to smoke it at to adjust for the drop when you add the meat) and expect it to take a bit longer than it normally would. That much meat mass will "absorb" a lot of heat and will take longer for the MES to recover and get back to the temp you want it to smoke at.
Be sure to take some pictures to show us!