I'm new here so I did not see your question until a year after you posted it. Sorry nobody bothered to reply so I'll give an answer in case this question comes up in a future search.
It is safe to say the number of swingers or folks in open relationships numbers in the millions worldwide. Even in countries like Egypt where that kind of behavior is illegal and punishable by death, folks get caught swinging.
Here in Canada, with a very small population we have over 80 swingers clubs listed and in the US there are hundreds of swingers clubs. Each of these clubs gets a few new members every week so you do the math....that's a lot of sexy like-minded people.