Thousands. There is no registration of such businesses, so there is no handy count. There are hundreds of bricks-and-mortar, SEO companies and thousands of home-based individuals. Then there are a lot of other people who claim to "do" SEO.
My abilities are in writing (copyrighting, technical, content) and SEM, SEO & PPC and my wife's talents lie in web & graphic design. We're not interested in looking for work at an agency or anything like that, because this down economy is a great opportunity for people like us to contact small businesses who don't want to pay ridiculous amounts of money to get the job done. It may take a little longer with a two-person operation such as ours, but we do quality work and we know what we're doing.
I think there are a lot more people out there like us, who want to take on just enough to continue to do quality work and meet our own personal benchmarks. I'm interested in making enough money to live comfortably and we already do that, so from here on out it's about what kind of work I want to do. It's nice to be in that position for a change.
There are so many people in our field, I prefer not to think about it. Doesn't really matter how many there are if you're good at what you do, because you'll float right up to the top anyway. Keep a good attitude and a healthy determination and you'll be fine.