How much do I need?

Figure 50% yield on both butts and briskets. Your actual yield may be higher, but that give you a buffer.

Is it self serve or will someone be serving the meats?

If it is self serve, people always take too much so plan on 1/3 lb per person. If someone is serving and you can control the portions you can plan on 1/4 lb per person.

So... 400 servings @ 1/3lb each is 133 lbs cooked meat, so that means 266 lbs raw.

400 servings at 1/4 lb each is 100 lbs cooked or 200 lbs raw.

Now, the trick is to decide how much of each meat. I don't think there is a magic formula for that. I'll let the guys with more experience comment on that, but my inclination is to cook enough for everyone to have some of each.