How is dianetics therapy to cure mental disorders, how many sessions dianetics can cure a person with depression?

Scientology. Dianetics is a type of intervention prescribed by the church of scientology, not physicians. L. Ron hubbard wrote about dianetics, and it has a large and cultish following. the church of scientology often pickets american psychiatric association meetings. You can consider how effective it might be by reviewing the behavior of famous scientologists -- easy to google.
Propaganda. I would look for real proof and not look at propaganda. Is the cure reproducible? Is the cured person able to describe the history of the illness and to relate how the cure happened? There is no medicine and no reproducible science in much of their claims. Be cautious like you are when other groups make spectacular claims of magic or miracles.