This mistaken question is often asked by people wanting some quantitative measure of the efficacy or track record of the impact of botanical medicine for their diagnosis.
"Success rate" is not a recognized term in cancer statistics. There is population data on "survival rates" or in more precisely on "disease free survival" or "progression free survival", usually for a time period such as 5 years. These numbers are broadly useful to help understand your diagnosis and associated prognosis. In some scenarios there may be sufficient clinical trial data about a particular intervention or treatment of a given stage of a given cancer that one can use survival rates to help decide on a treatment plan. For example, treatment Z may have more chance of increasing survival than treatment Y, but X has terrible side effects and Y has only mild side effects. The balance of risk and benefit in such cases may be aided by detailed statistics about that treatment.
A moments reflection should reveal why the question is inapplicable to the practice of clinical botanical medicine: Firstly, there is no clinical trial data that compares survival rates with and without botanical medicine. In a private practice setting every patients is unique and cannot be incorporated into population statistics with any accuracy. The only relevant data that we can gather relates to patient mortality, and we simply do not have sufficient data across different diagnoses to compile meaningful numbers in this regard.
What we can say, on the basis of clinical experience, is that botanical medicines, when used with expert guidance, tend to increase both the quality and quantity of life of people with cancer, which is why we are "successful" so much of the time.