if he is human number 3, it may have registered another 2. but I don’t know if other humans went to this world and were unable to register
Well, I suppose that with the information so far we can't prove or disprove that statement.
Regardless of it, you are right, there may be other ones hiding, but we won't know until more info goes up, besides, the creator of this game seems to like to put subtle stuff in the art and writing, so maybe we may find some subtle info lol.