Deep cleansing breaths. Prime rib is incredibly forgiving. No worries.
Bone in,? Bone out? Eye only? Includes the lip (that little strip of fatty meat that floats above the round)?
Bone in, lip on 11 pound roast is 5-6 bones. At this size, the roast is significantly longer than it is thick (from lip to bone), which means roast time is not strictly per pound but based more on time it takes for meat to penetrate from cap to center. I prefer to start in a hot oven but finish moderate. Preheat to 450, put the meat in and turn the dial down to 325. Cook time to dead rare, 120F, is around 2-1/2 hours. A long rest, 20 minutes at least, wrapped, in a prepped cooler. More to the point, the roasts may be held, foil wrapped, several hours in prepped Cambros if you have some or regular beverage coolers if you do not. Carry over won't take you past medium-rare and will give you good texture and temperature equilibration.
The only seasoning you're going to get on the roast will be about 3 miles from the center. If it's a lip on roast, the seasoning will be attached to an especially flavorful and rich piece of meat which can handle many tons of seasoning. If it's lip off, you may want to hold back to a few tons. In either case, don't be shy with the seasoning. I use the same basic basic beef rub I use for most beef roasts and steaks. I.e., kosher salt, black pepper, smoked paprika, granulated garlic, granulated onion and a very little thyme and sage. Large grain salt such as kosher salt is muy importante, mija.
If bone in, the usual allowance is 2 people per bone. With most groups, this usually means major leftovers. You can try to squeeze more out, but I'd play it safe. If bone out, expect 20% waste, figure 8 - 12 oz per serving depending, which yields about 14 servings per roast.
You CANNOT hold sliced prime rib in a chaffer unless your clients ordered pot roast. If that means a carving station, well dayum. You knew the job was dangerous when you took it.
A quote au jus unquote gravy will be expected. I find the simplest is 2 or 3 parts stock combined with 1 part red wine and cooked off just until the wininess disappears. You can further season this with a little onion, garlic, and/or parsley but strain before serving and serve au claire. Hold the jus as hot as possible and ladle it generously to carry some heat back to the roast.
Hope this helps,