I had this question posed to me recently, and I told the person I didn't think there was any such limitation. (You know, the way Facebook limits your friend count to 5000?).
But would be nice if there was a concrete yes or no answer on this. Personally I believe it's pie in the sky thinking on my friend's site. He wants to create a membership type directory with authors as members and wants to know if several thousand "authors" will break the site. I say no.