Two, one at the base, one at the joint in the arms.
Take a look at the web site and you can watch a great video demonstration to get a better idea of the specifics.

Lungbuster, I talked to Skip about his Rock Steady arm before I found the 3rd Arm. I think he has one of the best arms out there for what it is, which is a CNC milled aluminum arm of the type most pro shooters will be very familiar with because of the design since it's been copied so much by others. However, at a price tag of $600 without a headhis product is in a price range that is out of range for most budgets, including mine. I still think he has one hell of a product and I like the guy but he has no mass production capabilities.