That's one ticket for two games, starting at 12:10 PM on Thursday. Jon Niese will start tomorrow night, while R.A. Dickey and Chris Capuano will split the doubleheader.
Added by Chris: Presumably, if you had a ticket to the game tonight you'll be able to exchange it for a ticket of equal value to another game. In the past, the Mets even issued refunds in situations like these, but I'm not entirely sure if they're still doing that.