Most sources I've seen state that it can take anywhere from 500 to 800 stings to kill an average person. If you happen to be highly allergic to a component of the venom, however, all it takes is ONE.
I clearly remember the first time I was ever stung; I was 11 and visiting the house of a close friend whose father kept honeybee hives in his yard. My friend and I were invited to watch as the father checked on the hives in his beekeeper's suit. Even though we were standing 15 feet away and not behaving in a threatening manner, a bee somehow managed to get tangled in my hair and stung me on the back of the neck. Just bad luck.
Several years later I was stung again. I was mowing my parents' lawn and rolled the lawnmower over a hole that had been dug by a vole but had subsequently been taken over by wasps. Three of them stung me on my foot, right through the canvas running shoe I was wearing at the time. It felt as if I had been burned; my foot swelled and I had difficulty walking for two days. In retaliation I had someone come over after dark and set fire to the nest to destroy it, and then fill the hole with crushed stone.
The second incident in particular caused me to develop a fear of wasps. I know that they won't sting if you leave them alone. However if I happen to see a wasp within three feet of me I freak out, and sometimes even run if there's an avenue of escape. I won't visit a certain friend's house in the country during the summer because wasps and hornets nest in the gables despite her efforts to remove them.
At home I spray repellent in all the weeping holes in the brick exterior, and hang a fake nest on the fence. Many types of wasps are territorial and if they see what looks like another nest, they won't come near. So far, that seems to have worked.
Now if I can just get the neighbour's cat to stop using my flower garden as a personal litter box.