Jeff,
BB's are the ONLY ribs I cook so hopefully I can help you at least a little bit. I've cooked hundreds of racks by now and I find that the average adult is satisfied with 1/2 a rack given a reasonable amount of side dishes. I know many adults can eat a whole rack, but my guests have rarely tried to eat so much. Usually if there are any ribs left over the guest take them home. I always plan on 1/2 rack per adult, but you may want more.
As for time, that is such a loaded question. Given near perfect conditions (no or light wind, no rain, moderate to hot ambient temps) I use a chimney of unlit with a few fist sized chunks of pecan and apple wood blended in (I have an 18 1/2 inch WSM) and about a half chimney of lit on top of that blend. I catch the temp on the way up (I use a clay dish and no water) somewhere around 250-275 (temp is not so important; high temps work for many people), and then load the smoker. On a near perfect day I would say that 4 to 4.5 hours would do it, but I never cook by time, only feel. They are ready when they are ready. If you absolutely have to have them ready by a certain time, prepare them to be done by 2 or 3 and double foil them in a cooler until 4.
By the way, the best thing I do for ribs in my smoker is put a little tin foil strip on the grate under the ends of the ribs, maybe 2 inches wide or so. This trick keeps the heat chimney that comes up the sides of the smoker from burning the ends while the middles get "just right". I've done this a handful of times with great success.
Good luck! And I'm envious of your weekend!