How many asteroids are near Earth that are large enough to destroy an entire city?

Here's all the info you could even need on this. More than you want probably. If you aren't scared when you are done reading it something is wrong. Keep in mind, the size is not the only factor on the devastation one of these nasty buggers could cause. Mass and velocity both play a factor. A heavy smaller object travelling faster is just as bad as a bigger but lighter slower moving object. The Reasoner -