Explanation

According to Einstein, increasing the brightness of a beam of light without changing its color will increase the number of photons. 

Einstein's theory of light posits that light consists of discrete particles called photons.

Brightness is related to the number of photons present in a given area over a specific time.

When you increase the brightness of a light beam, you are essentially adding more photons to that beam,

While keeping the energy of each photon the same (which is related to its frequency and color).