One estimation is about 7,000. According to Webster's, aster means star and eides means like. It was Ceres. Astronomers say they are mainly made of iron and stony materials. Ceres is almost 580 miles in diameter and Pallas is about 330.
Ceres is the largest known minor planet.
There are believed to be millions of asteroids the size of boulders in the solar system. Smaller ones, maybe a half-mile in diameter, may do so within the next 1,000,000 years. It is doubtful that the larger ones would, however. And of course, there is recent speculation that an asteroid could come close to hitting Earth within the next few decades. For an asteroid half-mile in diameter, maybe 8 miles long.