More than 20. We have stayed together due to allowing the Relationship to be Open near the beginning (not that I'd recommend it for everyone, it isn't but it worked for us) and then dedication, Love, kids, LOTS of sex, mutual respect, and luck.
The Open Relationship after about 6 months together probably saved us. I was young when I met him, he was a little older, but we both needed more experiences before settling down. We were Open for about 4 years, maybe a little more, then we decided we were both ready for Commitment. I was only 16 when I started dating him so I NEEDED to date other men. I was also a virgin when I met him. He had experience with women, but needed to sow some Wild Oats, in part, to prevent both of us from waking up years later and thinking, "What did I miss?"
We put a LOT of work into our relationship. We don't talk about "The Relationship" that much (he's a guy....most guys shudder if you mention the term "The Relationship") But we both feel free to bring up something if it's bothering us, and have learned when to shut the hell up about something that isn't that important. The hardest part is knowing what to bring up and what to NOT say anything about. That comes with experience.