Knorr used to make a packet mix that was tolerable, but it is discontinued. The typical modern sauce is a cream sauce with sherry, brandy and maybe egg yolk. You need about 2 1/2 cups and 2 pounds of seafood to make for 6 people. So, you would be looking at about 2 1/2 to 3 gallons sauce and 32 to 36 pounds seafood for 100.

The commercial sauces often have some tomato paste, or even sauteed shallots or bell pepper.

Seafood Newburg-style Sauce

3 cups butter
2 cups all-purpose flour
2 1/2 tablespoons paprika
1 gallon clam juice or seafood broth
1-2 quarts milk
Pinch of salt
Possible, 2 tablespoons old Bay seasoning
3 cups dry sherry, can be 1/4 brandy
1 quart cream, light cream

Melt butter over a medium low flame.
When completely melted, add paprika and stir for 2 minutes.
Add the flour to the butter and stir for 2 to 3 minutes to cook the roux.
Stir constantly to avoid burning.
Add the clam juice/ seafood broth and stir until thickening begins.
Add 1 quart milk, cream and sherry.
Simmer for 5-10 minutes and, if needed, add remaining milk to achieve desired texture.
Serve immediately or refrigerate for future use. Reheat in a double boiler or over very low heat.