It’s true that the majority of asteroids will be located right between Mars and also Jupiter. In fact, our solar system by itself is said to possibly have over 150 million asteroids - that’s kind of a lot.
Thinking officially here, there is no doubt that we have only counted a small percentage of asteroids. There are just over one million cataloged, but when you put this into perspective with the number of asteroids within the solar system, then this is a minuscule amount.
The brightest asteroid is called Vesta, a 525km wide asteroid, and this has orbited slightly more than just twice the Earth’s orbital distance away from the sun.