Reading Challenges -- How Many Is Too Many?

I discovered the group of book bloggers on the internet only last year. One of my favorite things about book blogging is the Reading Challenges that the book bloggers throw down. I quickly became addicted to them. This year, I am trying to use the Reading Challenges to help me read the books in my library that I have not read. But, how many challenges can a person actually complete. I am doing two right now, the BangBang and the Book Awards II challenges. And, I have already signed up for two more that begin in January. I am looking through my library for books to meet another challenge. I am making good progress on the two challenges I am reading for now, but I don't want to sign up for too many. Then again, how many is too many?