Bad Hosts: Invites you and, after you accept, asks you to bring dessert for everyone.
I've been invited and then told to bring something, which really bugs me! I feel that if you are inviting, you just need to make it clear if it's pot luck or just really invite someone, period without expecting anything.
My husband and I were working on the house one afternoon and a friend invited us to dinner that evening. So we accepted, and as we were busy we didn't offer to make anything. We always take wine anyway, which is accepted among our friends. But then she asked us to bring a green vegetable. I had not planned on stopping work early so that I could go shopping, come home and prepare food. So needless to say I was po'd. This same hostess has a habit of asking all the quests to bring things and then only making a miniscule main course. And if the guests bring side dishes that are too small we all go hungry. As a precaution we offer to make a huge side dish of potatoes or rice!
But as for your dessert, I don't think a host should expect dessert for that many people, just ask three or four people to bring dessert and let them know they are splitting it.
But I think it was unfair to ask for something like desert for 19 people! I agree with Patticakes that I'd say I can make enough for a certain number of people, and can anyone else make some? As long as you give the hostess enough notice.