At http://phys.org/print361438706.html … the composition of asteroids is discussed. They are mostly rock but some contain a lot of clay and silicates as well as various metals such as nickel and iron.
We are told that asteroids are the rocky remnants of the proto planetary disc of dust and gas that formed in the early solar system period, small dust grains that developed into small rocks and later, large rocks, the accretion model.
The elements to create water, and water itself, are present in asteroids. Some may contain water and ice in their interiors – even on the surface.
There are millions of asteroids in the solar system – one estimate is 150 million of them. However, just 13.024 are classified as Near Earth Objects.