Hundreds of thousands of asteroids have been discovered so far, about 20,000 of these have been given a name as well as a number to identify them. The largest, Pallas, is about 550 km in diameter. The smallest known asteroids are a few tens of kilometres in diameter, but we expect there to be millions more that are smaller than this.
Most asteroids are found in the Asteroid Belt, between Mars and Jupiter. Some asteroids are actually in the same orbit as Jupiter but they won’t collide with Jupiter, these are called Trojan asteroids. Other asteroids have passed very close to Earth. These are called Near-Earth Objects or NEOs.
If you stuck all the asteroids together, you would have an object that is much smaller than the Moon.