You will use about twice as much dark chocolate as white. Amounts depend somewhat on what other sweets are being served and what dippers you provide.

Look at price of bulk/ restaurant suppliers chocolate chips or dipping chocolate tablets for very large amounts.

The chafers are double layer with water in the bottom pan. If you try to do single layer with candles, it will scorch. Cooking heat is NEVER above medium.

You do thin it out some, there are several approaches:

This one can be made ahead and reheated-
Per 6 in buffet service, 4-6 at table.
1 cup semi-sweet chocolate chips (white or dark)
2 tablespoons butter or
1 (14 ounce) can Sweetened Condensed Milk (NOT evaporated milk)
2 tablespoons water
1 teaspoon vanilla extract (May sub 1 tablespoon brandy

Directions
In heavy saucepan, over medium heat, melt chocolate chips and butter with condensed milk and water. Cook and stir constantly until thickened, about 5 minutes. Remove from heat. Add vanilla or liqueur.
Serve warm as a fruit and cookie/cracker dipping sauce or drizzle over ice cream or cake. Store leftovers covered in refrigerator.

This one is day of, can be held in a double boiler. Serves about 16 on the buffet, 12 at the table.

Ingredients
2 1/2 cups half and half
1 teaspoon pure vanilla extract
1/2 teaspoon kosher salt
4 cups semisweet chocolate chips
1 1/3 c. milk chocolate chips

Directions
In a medium saucepan over medium heat, whisk together milk, cream, vanilla, and salt. When bubbles begin to form around the edge of the pan, remove from heat and add chocolate chips. Let chocolate sit for 5 minutes undisturbed, then whisk until smooth. Serve with dippers of your choice.