I just did my first one for 14 people and had leftovers. Timing everything out is very important.
10 lbs shrimp
8 lbs andoullie sausage (cut into shorter pieces)
frozen corn (small cobs)
10 lbs potatoes (smaller)
Bread (we made garlic)
2 1/2 gallons water
2- 8 oz Louisiana liquid concentrated crawfish, crab and shrimp boil
4 lemons cut in half
6 small whole onions skinned
(I let all the food items come up to room temp so I could get a quick recovery on the boiling water.)
Add concentrate, onions & lemons to the water and bring to boil.
Add potatoes keeping in mind size will dictate the length of time needed to boil. (usually 15 minutes or so before adding anything else)
Add corn and sausage
Add shrimp/crawfish (these actually only take a couple of minutes to fully cook.
If it were just me I'd would add something to spice it up more. Maybe more concentrate. Good luck with your boil!