There are 5 spots in all. Get a slot, while there is still one left.
Characters can be child or adult in age but must be in a costume of some kind. If you are interested, reply in his journal with a character reference and what you want your costume to be. If you cannot think of a costume but still want to be in the image, you can work something out together with him. He limited the number of slots to 5 so he can finish this before Halloween.