Is there any humor or meaning to the conversation about Yuri's dream and how many Hinatas there were?
The joke here, I think, is that before this conversation, Hinata showed up in front of Yuri and behaved very differently from yesterday (before Yuri dreamed about him), so Otonashi said that the warning came true: Hinata really changed. Since Hinata said, "don't think that the me from today and tomorrow will be the same", tomorrow, he might change again, making Kanade say that there could be more than three of him.