Yesterday, some guest came to my home. I served Biryani and Chicken Quorma to them. And I served apples as dessert/as a dessert.For a meal eaten at home, it's not uncommon to have more than one dessert (either different people have different desserts or some people have two desserts), but it is uncommon to have more than one main course.
We're having tuna casserole. For dessert, there's cake, cookies, or fruit.
"Main" pretty much defines it as being singular.