I don't believe for a second that as many people like BDSM as is represented in pornography.
A minority of pornography is BDSM-oriented; most porn performers who start out in BDSM (because that's their inclination) branch out into mainstream porn because that's where the money is. AFAIK, there are only two major BDSM studios. Finally, a scholarly estimate, from the introduction to Sadomasochism: Powerful Pleasures, edited by Peggy J. Kleinplatz and Charles Moser, 2006:
We have estimated privately that approximately 10% of the general population is involved in SM, but there are no studies on which to base that figure. Based on our experiences in conference hallways (where the real exchange of knowledge and learning in academia often transpires), an even greater proportion of sexologists, sex educators, sex researchers, and sex therapists are so involved. Although sexologists are usually quite nonchalant about their personal sexual behavior, those who have confided in us have done so in hushed tones and have requested our undying pledges of discretion. We have even run across partners who each request our secrecy, but cannot bring themselves to tell one another. Simple self-disclosures are not typically sufficient to solve the couple’s problem; such are the intricacies of SM. In some instances, both partners are dominant, or submissive, or maybe one yearns for the physical aspects of SM while the other for the psychological aspects, or maybe their familiarity destroys the fantasy, etc.