Gumbo for 20 People

Kind of depends on how many women, kids and what else are you having with it besides rice, etc.

Little kids won't eat as much, if they eat it at all, but teenage boys can destroy it.

I would say you'd be safe with 3 gallons worth. Make your stock from the shrimp shells and heads if buying fresh shrimp. The gumbo crabs will add a good bit of flavor.

Oysters would be a good addition, along with the oyster liquor if you shuck your own. I will often use bay scallops in my seafood gumbo too. Those are the smaller ones about the size of the end of a fingertip and if you use them, add them late since they cook fast.