Electrons and positrons in turn may produce more gamma-rays by the bremsstrahlung mechanism. The number of particles starts to increase rapidly as this shower or cascade of particles moves downwards in the atmosphere. On their way and in each interaction the particles loose energy, however, and eventually will not be able to create new particles. After some point, the shower maximum, more particles are stopped than created and the number of shower particles declines. Only a small fraction of the particles usually comes down to the ground.
Actual numbers are subject to large fluctuations.
In fact, from most cosmic rays nothing comes down at all. Because the earth is hit by so many cosmic rays, an area of the size of a hand is still hit by about one particle per second. These secondary cosmic rays constitute about one third of the natural radioactivity. When a primary cosmic ray produces many secondary particles, we call this an air shower. Most of these particles will arrive within some hundred meters from the axis of motion of the original particle, now the shower axis.