There are two different questions that often get mixed up: "What can hypnosis do in general?" and "What can hypnosis do for me?"
What's possible in general
In general, quite a lot is possible — some people can be hypnotised deeply enough to sit in a tub full of ice without feeling the cold. That shows what humans are capable of, the same way it's true that humans can run 100 metres in under 10 seconds. Both statements are true, but only for a handful of individuals at that level, who also usually bring extensive training or unusually high hypnotisability with them.
What's realistic for you personally
For you personally, that means: most people can draw noticeably more out of themselves and get closer to their own potential with hypnosis, without ever needing to reach those extremes. In practice, I see this mainly in three areas:
With habits and behavior — quitting smoking or weight loss, for example — we work on consciously interrupting automated actions and replacing them with new ones. This is one of the areas where hypnosis tends to work most reliably, because the pattern is usually clearly identifiable.
With emotions and reaction patterns — fears, stress, old hurts — the work is about reshaping how you react to particular situations. Progress here is often less absolutely measurable, but just as noticeable in everyday life.
With physical processes like pain or tension, we make use of the fact that body and nervous system are more closely linked than most people assume. Here too: hypnosis doesn't replace a medical work-up, but it can noticeably strengthen your own regulatory capacity.
What you can actually achieve ultimately depends less on some general limit of hypnosis than on how clear your concern is and how willing you are to engage with the process.