According to the Bad Pitch Blog (love them!), the answer is between and three and five pitches. And sometimes it can take up to a year to work a good story in a key media outlet.
Based on Kevin Dugan’s anecdotal evidence, this post does a great job of giving you solid guidelines for managing client expectations for a media relations campaign. I suggest you save it, bookmark it, print it, tattoo it somewhere, print it, whatever.
I have certainly had clients that, despite my best efforts to manage expectations, never understood why the ratio wasn’t 1:1 (pitches:stories). I love Kevin and the Bad Pitch Blog for laying this out so simply.
The Bad Pitch Blog: The Math Behind Good Pitches