A 40 qt. stockpot will give you 1 qt. of broth per person, counting the seafood. That's a lot. People will generally eat about 2 cups of cioppino so you could cut that 40 qts. by half and have plenty. I'm assuming there will be other food to serve. I have some problems with people bringing their own seafood to add. Seafood stew is an iffy thing to deal with given the problems that can result if the seafood wasn't handled properly. I worry about these things, since other people will be contributing the seafood. I make a lot of cioppino, I love it and this sounds like a great party. Have fun.