Q. How many letters are there in the correct answer to this question?

A. Four.

this is a self-referential construct, as are doc_comfort's examples.

Q. How many consonants are in the correct answer to this question?
A. one
or,
A. two
or,
A. three

this is a variation, of sorts, on the "this sentence" theme:
1. This sentence contains five words.
2. This sentence no verb.
3. This sentence is false.

-joe (autological) friday