The Max Ray Depth parameter can be found in Render Settings > Corona > Performance Settings. It controls both the maximum number of global illumination (GI) bounces and the maximum number of reflection and refraction events a ray can undergo.
The default value of 25 means that each ray can intersect with scene surfaces up to 25 times, whether through diffuse bounces, reflections, or refractions. For example, when a ray passes through a solid glass object, one intersection occurs when it enters the surface and another when it exits.
Increasing the Max Ray Depth beyond the default value typically does not produce noticeable visual improvements.
Examples
Crystal - lowering the Max Ray Depth value results in less internal reflections and the object becoming generally darker.
1.1. Crystal - MRD = 25 (default value)
1.2. Crystal - MRD = 1
1.3. Crystal - MRD = 2
1.4. Crystal - MRD = 3
1.5. Crystal - MRD = 100
Two mirrors - Max Ray Depth defines the number of mirror reflections with the value of 100 creating an "infinite mirror" effect.
2.1. Two mirrors facing each other, camera between them - MRD = 25 (default value)
2.2. Two mirrors - MRD = 1
2.3. Two mirrors - MRD = 2
2.4. Two mirrors - MRD = 3
2.5. Two mirrors - MRD = 100
3.1. Room - MRD = 25 (default value)
3.2. Room - MRD = 1
3.3. Room - MRD = 2
3.4. Room - MRD = 3
3.5. Room - MRD = 100