Interesting, only 1 billion bicycles? I would have expected more than that. In most of the places I lived (Europe, Asia) there seems to be more than one beater bike per person lying around somewhere (and some nice bikes, too, but much fewer).
Many (most?) children have bikes, and lots of elderly people have some bikes somewhere in a shed back from when they used to be able to ride.
Almost everyone can afford a beater bike. That's why they are so common in the third world.
I'm not sure how out of shape you need to be not to be able to ride a bike? In any case, I wasn't wondering about how many people actively ride bikes. I was wondering how many bikes there are.