Three. The first was my first bicycle - a 20" red Schwinn which I left unlocked outside a grocery store while getting a candy bar in the mid '50s. We reported it stolen and it was recovered a week or two later when someone left it in the street and someone else ran over it (broke some spokes) and reported it to the police. The second was an English 3-speed about 10 years later. Also left unlocked - this time outside a campus building while I went in to visit my father. The third was about 10 years after that. A 10-spd Schwinn Letour kept in a closed, but unlocked storage shed. None in the 35 years since then although I do frequently leave my bike outside stores and other businesses with just a cable lock.