I do not charge for signatures. I never have, and I do not plan on starting at the show this summer.
As for limit - I'll sign just about anything I've written, no matter how great the stack of books; all I ask is that, if there's a line and you have 100 books, you let me work in batches, so other people with smaller stacks can have their books signed, as well.
Very much looking forward to attending the show!