I've been looking into this kind of thing recently; I just cooked 24lb of pork shoulder for a cookout at work for roughly 40 people.
The rule of thumb I've been seeing is to assume 1/2 pound of meat, before cooking, per person. Once cooked, you will lose about half of the weight, so you wind up serving about 1/4 pound per person. My experience is that it is enough to generously cover a hamburger bun for a barbecue sandwich per person.
If you have big eaters, you might want to increase this a bit.
So, doing the math: 20 people => 10lbs of brisket. Around here (Ohio), the brisket flats tend to run 6 to 8 lbs, so I would say you need two brisket flats. This will give you some extra meat, in case you have the big eaters mentioned above.
As for space, you've got plenty in the WSM - one brisket flat per grate.
However, if you need more space, check out the
Increasing Cooking Capacity page on the main VWB site for ideas on how to add more space for cooking. I have one of the Big Green Egg grid extenders mentioned there, and I love it.
Hope this helps!
