Is a track in a cloud chamber made by one particle or many particles? Is the track in a cloud chamber made by one particle interacting with many particles each emiting a photon or is the track made by many particles interacting on the same trajectory? or is it a cascade of interactions?
 A: You are asking about a cloud chamber, not a bubble chamber. The processes observed are similar to both, the technique is different, the bubble chamber being a very precise and sophisticated instrument. 
Here are some tracks in  a cloud chamber:


The general procedure was to allow water to evaporate in an enclosed container to the point of saturation and then lower the pressure, producing a super-saturated volume of air. Then the passage of a charged particle would condense the vapor into tiny droplets, producing a visible trail marking the particle's path. 

The picture on the left is an exposure to a radioactive source, radium,  and each track is an alpha particle ( an ion of helium4).

The device came to be called the Wilson cloud chamber and was used widely in the study of radioactivity. An alpha particle left a broad, straight path of definite length while an electron produced a light path with bends due to collisions. Gamma rays did not produce a visible track since they produce very few ions in air. The Wilson cloud chamber led to the discovery of recoil electrons from x-ray and gamma ray collisions, the Compton-scattered electrons, and was used to discover the first intermediate mass particle, the muon.

What is happening is analogous to the contrails from the jets seen in the sky when the conditions are right. It is the signature that a charged track has passed.
Little bubbles form where a single charged particle has passed. As it passes through the air it ionizes the atoms within its path by soft electromagnetic interactions and that is where the water condenses leaving small bubbles that define the track. 
