The people I go boating with have about 30 life jackets. I think it's unreasonable to ask someone to come boating with you, but have to bring their own jackets.... Seriously. It's one thing if you were another boating person that would obviously have life jackets, but dang.
Hey, at least they let you know ahead of time. I would expect them to have adult sizes for every seat and then maybe a few extra children's or adults. Anything beyond that probably isn't unreasonable for them to request you to bring. Remember, you don't have to go....