In the U.S., anyway, "panini" is consistently used as both singular and plural in every place I've been that sells them. I tried ordering, "I'll have a ... panino, please" a couple of times, and got a strange look each time. By the way, again at least in the U.S., they aren't crispy buns, they're warm sandwiches on sliced bread pressed flat in a grill press. In Italy, I remember the word referring to a cold sandwich on a small baguette.