As I didn't know you receive one for reaching 1st every week and remember someone saying (I think it was bitcoingirl) who mentioned we needed 18 games correctly predicted to have a hat donned on you.
There are no magic numbers regarding the yellow caps and every tournament is different based on the results people get. You could get a yellow cap with a mere point if everybody else failed to pick a correct outcome. You could end up with more yellow caps than anyone else but still not win a paid slot if your other round results aren't great as it's all about consistency. Winning a few yellow caps won't mean much (other than the free bets) if you're not consistent throughout the league.