Can I reuse 3D printed parts? I need to show a demo to people on a daily basis. But every demo is for a different 3D printed part. 

Can I reuse the material? Is yes, what is the maximum number of times that the material can be reused?
I ground up some prints and extruded them into new filament. The quality of recycled filament is not very high so I would not recommend it. 

As pointed out earlier, the shredded bulk material you would create from your old prints would cost more in time and energy to produce than buying new raw material (pellets, not filament), and you still need to extrude it into filament to be able to use it in a printer.