Can us mangaka add more than 1 character in the dream collaboration?(for random or something....)

That is something I'd honestly like to do a little later. Maybe after the first-come-first-serve characters are introduced or maybe earlier.

Perhaps later, but not now. Since we have 10 people participating in the manga, we have at least 10 main characters. For minor characters (like passer-bys and students) you can just draw them randomly.

-Ki