I'm making mini cream pies in four custard cups how much cream should I make?
It totally depends on your recipe and the other ingredients you are using.
This is probably more complicated than you'd like to hear, so feel free to try something else, but here's what I've done. I look at the recipe that calls for the full-sized pie, and look at what type of baking/pie dish they call for, and then I look up how much that holds. For example, a 9-inch pie pan, which is what most pie recipes call for, holds 4 cups.
Then I look at the dish that I want to use, and I measure how much it holds. I fill it up with water and see how much water it takes. Let's say it holds 1 cup. Then I would make the original recipe just as written, and divide it equally between my four custard cups. Let's say they only hold 1/2 cup each, and I have four of them. Then I would divide every single ingredient in the original recipe by two, and then divide it equally between my four custard cups.