Amazing, isn't it? Most people count three of them, so much for perception, but it does not say anything about your IQ. Whoever counts the six 'F' the first time is a genius, four is rather frequent, five is rather rare, three is normal. Less than three, and you must change glasses ; -)
For some obscure resaons, we (or our brain) do not count the "f" in "of", maybe because the phonetic is similar to "ov", or because during (quick) reading the brain focus on "lexical" words, and not so much on "grammatical" words.