You need bout 9 1/2 gallons if you are serving it, about 11 1/2 if it is self serve and the only soup.
I add an Oriental soup base called Dashi, or dashi no moto to my chowder, instead of salt. About 1 tablespoon per batch for 6. Find it at an oriental food market or the oriental section of a big supermarket. I do use clam juice.
Canned clams are available in a large 28 ounce can at restaurant supply stores. 1 can per each 10-12 people.
This off site ref would be useful to you: