You probably already knew that a Panini is a grilled sandwich with Italian origins. But did you know that the plural of the word "Panini" is actually just Panini? There’s no such thing as the word “Paninis!”
You pronounce it with the emphasis on "PAH-nee-nee" if it’s one, and pronounce it "pah-NEE-nee" if it’s two or more. It's as simple as that.