The answer is four adult cats. It is a big squeeze, though. They have got to be friendly because they are all over each other.
We know cats generally like boxes. We know cats have adapted to getting along in colonies and supporting each other. But, this is getting silly. These guys have got to be the best buddies to be climbing over each other to book a spot in this very desirable box.
About the picture: It is from the pictures of cats.org Facebook webpage. I believe it is free to use.