There is no set number that anyone can tell you in advance. You just have to get started and see how colonics help you. Everyone is different, and if someone tells you on your first visit that it will take 3 colonics, or 6 or 12 to reach the point of releasing plaque, they are not being honest.
My suggestion is to plan on at least 3 sessions as a starting point, and then revaluate your program at that point. By then you should either have a good feeling about your therapist, or perhaps be skeptical that things are not working out.
Colon hydrotherapy is not an exact Science in which results can be predicted with accuracy in advance. But in general, for most people a series of colonics can be very helpful.