Not that much I guess. You'd be surprised how much of them are still used frequently.
There are of course... but they are left there for other people to ride with. Some of them are stolen and left there too, for others to use for free.


Leuven en Ghent are student-cities.
Most of these bikes are not there for the regular citizens who live there, but for temporary students who are there for 1 to 5 years max, or for people who need to use it for a few weeks or months.
Those bikes are mostly *nd-hand bikes, you buy or lend it for a few dollars from the old students who leave the city, and when you finished university, you give it away to the next generation students.

Lots of bikes are stolen too, some students get their bike stolen by another student, and then steal a bike from others too. Therefore, some bikes are there without locks, so you can just pick one up and ride, leave it where it is in the same city. You find bikes all around the city so you actually do not need locks. You just hop on a bike and ride.

Most adults, the working class, have mostly better bikes and store them at parkings that are monitored against theft by video camera's. They lock them of course.