Many problems, especially most phobias are gone after one session. But, I always recommend that clients have at least two sessions. In fact, I used to bundle a second session along with the Introductory Session every time, but in thinking about that, it detracted from the no high-pressure upsell that is my policy.
But I still strongly recommend at least two sessions for everything.
Very often, a new client, especially if they have never been hypnotied before, spends much of the first session not wanting to "miss anything", and thus they don't relax as completely as they could. That's normal. However, in the second session, the client knows what to expect and will "let go" more, so they are able to go into trance deeper and more completely.
So, while one session may be enough for relief from some issues, I ALWAYS recommend a follow up session. But it's your mind and your wallet, so I am not going to high-pressure sell you on that.