Rule of thumb is 1/3 of a pound per person. You will need 33 pounds of cooked butt, which means 66 pounds of raw. Depending on the number of racks on your uds, you might be able to do it. I cooked 15 pounds of raw, and it took up less than 1/2 rack. I have two racks in my UDS, so it might work.
BTW: If you aren't familiar with that pig cooker, you are inviting Mr. Murphy and his gang to eff up your cook. For a first time event, I would dance with who I brung....
Dbl Bbl Smoker, UDS, ECB, Char-Broil gas grill & charcoal grills, SJ - Certified Moink Baller