A free-standing egg custard has a recipe ratio of 2 to 1 liquid to egg by weight (a kitchen scale comes in handy here). So a quiche containing 2 cups (or 16 ounces) of liquid would contain 4 eggs (or 8 ounces, an egg weighing about 2 ounces each). If there's too much liquid in the custard, it won't set well. You'll end up with possibly a very soft or even runny quiche.