There's quite a few plushies, and I know that there are 5 of them for a minimum, but I'm not sure how many in total.
You can get one for each girl, by beating her at the maximum level of difficulty. So in order to get Bonnie's plush, you'd have to have her at 20 for a difficulty. For golden freddy, you have to enter 6 9 6 9 for the custom difficulty for night 4/20.