I think the formula is you are allowed one sexual ‘fuck’ or three non-sexual 'fuck’s. This formula was pretty rigidly followed (whether or not it was official) until the release of ‘My Best Friend’s Wedding’ which was originally going to get an R for more than one use of the word ‘fuck’ with the sexual meaning, but the producers successfully lobbied for a PG-13.
Interesting story I heard about the word ‘fuck’ in movies - ‘4 Weddings and a Funeral’ was the first made-for-TV movie in the UK after they relaxed their laws to allow the use of the word ‘fuck’ on TV. There was speculation as to whether the word would be used in it and to what extent, and that is why the dialogue for he first few minutes consists of nothing but the word ‘fuck’ or variants thereof - and then the rest of the movie is fuck-free.