A mechanic will have maybe one car running at any one time, maybe more if there are other mechanics in the same shop, and only for 8 hours a day. If you live near a road you'll have hundreds of running cars in your vicinity 24 hours a day.
General car mechanics probably not. I don't think they spend that much time next to running (and not just idling) engines. But I like that train of thought... how about professional drivers - truck, bus, taxi?