One. It was a slow motion affair at a stop, and it was all my fault because I didn't notice the guy in front of me was turning right and we kissed wheels. Fell over. Put his real Campy Proton out of true and so thoroughly screwed my front wheel that I had to have a new one built [old hub and spokes, new rim]. I rode 35 km home with a front wheel so wobbly that I had to open the brake [not just from the QR, either] almost as far as it would go.
No one was seriously hurt, but it cost.