Well, that's kind of hard to say... Most people in the "vanilla" world give at least a little lip-service to some very light BDSM activities at one point or another... Be it hair pulling, ass-slapping, dirty talk, that sort of thing...
If your question is more aligned toward how many people are in committed BDSM relationships (Dom/sub, Master/slave, Top/Bottom, etc), where kink is more of a common item in their relationship, then I'd say I have no idea.
I know that we members of the kink/fetish community are more common than many vanilla people know, but I also know that many of us find it hard to be honest with the "normal" people around us (with regards to our sexual activities and interests), as many of the people we know might treat us poorly (or at least differently) if they knew our degree of kink, or the degree of our involvement in kink or fetish activity.
I'd be willing to say that probably 3/4 of the sexually active adults pay at least lip service to BDSM activities (the "light" BDSM stuff like butt slapping, hair pulling and the like). For those who are into more kinky activities, I'd guess at perhaps 30%, but be aware that all I'm offering here is a SWAG (SWAG = Simple, Wild-Assed Guess).
Interesting question, though. I'm in a D/s relationship, so I know we'd be considered part of the BDSM lifestyle.