A root canal is a treatment inside where the nerve or pulp tissue of a tooth has been extracted due to a pathology. The space is cleaned up & sealed off allowing the body to restore the area around the tooth back to its normal healthy state by disinfecting the area and entombing bacteria & contaminates inside your tooth. How many visits? Either 1 or more. If 2 are needed the 1st involves cleaning & shaping the root canal system leaving some medication inside your tooth to aid in disinfection. The 2nd is 1-3 weeks later for filling & sealing leaving time for the interior disinfection to take place. A single-visit therapy is possible today due to advancements in dental science without compromising the quality of the work, although with multi-rooted teeth, such as molars, it may run into 90 minutes but there is less risk of bacterial leakage from the temporary 1st restoration. Single-visit is best when the pulp is still partially vital. When the patient is in pain or swelling this indicates a more complex case & so a single-visit is not recommended.