First, let me welcome you to the SMF family! You may want to drop us a post in Roll Call and intriduce yourself so we can get to know you better, thereby aiding us in your assistance in the future.
Not knowing your previous experience with brisket, I would have to consider a few other things here to be sure you're on track with having enough meat for everyone, and also to give you a bit more insight to other options, given the versatility of the cut of beef you're dealing with:
1) if smoking a full packer, with little or no trimming of the fat cap, and then seperating the point/flat muscles and trimming fat, you will have about 50% loss;
2) if smoking a center cut (flat only), this will be trimmed-up nicely, with little to no fat, giving far less loss after cooking (approx 20-30% by weight);
3) if you plan on making burnt ends from the point as an appetizer or side for the meal (we love these), you will have far less for slicing with only the flat remaining for sandwiches.
Another option which is favored by many is to seperate the point/flat after smoking the packer, trim the fat and foil each cut seperately to wrap in towels and rest. After resting, slice the flat and pull the point. This give the best of both worlds for sandwiches, as some may like to have sliced, and others pulled meat.
The reasoning behind my mention of burnt ends or pulled beef from the flat OOPS!!! EDIT: point is that the point can be difficult at best to slice if internal temps and/or resting periods are excesssive. The point has much more intermuscular fat than the flat which adds to it's natural tenderizing from cooking/resting. It may just shred under the weight of your blade when trying to slice, and if that's the case, just go for pulled...you won't be disappointed in the resulting product.
I think the brisket must be my all-time favorite for smoking and dining, just because there are all these differing ways to finish the product. Well, that, and it tastes so darn good!
Lastly, don't be afraid to make more than you know will be eaten the day of your gathering...leftover brisket makes great lunches!
Have a great smoke and a wonderful weekend!
Eric