Let me start by saying that hypnosis is not a miracle cure, even though that it sometimes seems to be. There are cases where the client is cured from alcoholism, drug addiction or something similar in one session, but that's definitely not the norm. Therefore you shouldn't be disappointed if you addiction isn't completely gone after one session of hypnotherapy.

The motivation to quit an addiction could very well be a great part of the chances for success. Some people use hypnosis as the last resort, but they want to quit the addiction so badly that they will try anything. Such people are very motivated to quit the addiction which makes the job of the hypnotist somewhat easier. These could be the cases where hypnosis can help in just one session. Others consult a hypnotist because they feel obligated to do so by a family member or friends. Such people aren't really that motivated to quit and are therefore harder to work with.

This shouldn't be seen as a dichotomi of wether your addiction can be cured in one or more sessions. The complexity of the addiction and the substance of addiction also has something to say. You can ask the hypnotist you call how many treatments he will estimate that you need, but it will be a total guess, since your case cannot be estimated on the basis of others, every client is unique.