Some good points.

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.