Potentially Hazardous Asteroids (PHAs ) are space rocks larger than approximately 100m that can come closer to Earth than 0.05 AU. None of the known PHAs is on a collision course with our planet, although astronomers are finding new ones all the time. http://www.spaceweather.com/
On December 12, 2008 there were 1010 potentially hazardous asteroids.
for more information on Near Earth Objects - http://neo.jpl.nasa.gov/neo/