If you'll bear with me, I have just picked up Kernighan & Ritchie "The C Programming Language, 2nd edition, ANSI C" from my shelf, and will have a look to see if there is anything in there.
Edit: I'm not really sure that there are any limits imposed by the language, though there might be in the implementation of the compiler that you use, and of course, the machine itself.
Edit2: Appendix A12 (p228-233) Preprocessing: says nothing about limits.
Edit3: Chapter 4.11 (p88-92) The C Preprocessor: says nothing about limits.