zlacker

Only a small fraction of the 1.2 million are potentially concerning, most known asteroids are main belt asteroids between Mars and Jupiter and will never come dangerously close to Earth. They are all visualized here: https://eyes.nasa.gov/apps/asteroids/

Observing near earth objects (NEOs) requires shorter exposure times because their (apparent) motion is quite fast, and that has an impact on how faint the objects you can detect can be.

There are methods like synthetic tracking though, that can detect fainter objects (even those hidden in the noise, not visible by eye).