One and only
Dean knows, really, that some day they'll probably have to shut the gate to heaven. Angels, man. What can you do? But for now things are sweet. He has a home, Sam's safe, and Cas, well, he's Cas. And if the reason Dean's putting off slamming the door on their feathered neighbors is that he doesn't want to lose Cas when it took so long for them to finally get their shit together, well, Cas is going to leave him one day anyway when he finally works out what a loser Dean is. So if Dean can put off closing the gates until he'd get all the good times he can, that's not such a bad thing, right?
So when hostile angels decide to cause a mess, the Winchesters realize they need to close the gates of heaven once and for all, but they're going to lose Cas too unless Dean can prove himself worthy.
The Curse of a Hundred Loves