How many asteroids are there in the asteroid belt?
Short answer: Probably a couple of million.
Long answer: It's currently impossible to give an exact number as there will be many asteroids that we haven't found yet. There's also the question of exactly how big a piece of rock has to be before we call it an asteroid.
We know of over 200 asteroids that are larger than 100 km in diameter, and up to 1.7 million asteroids that are larger than 1 km in diameter. Of course there are sure be be many more smaller ones, with millions of objects in total.
The inner Solar System. The main asteroid belt is shown in white, with other asteroids in red, orange and green.
Image by Mdf