Really good question and glad to answer. When I was still in training (residency and fellowship in Interventional radiology) I performed procedures under the direction of the attending doctor (the professor) so I had done X number of whatever procedure when I first got into practice. But as time goes on new procedures come out. If the procedure has parts that are familiar, it may be easy to learn the new additional technique. Example:. I had performed hundreds of angioplasties of arteries (in leg, kidney etc) in my fellowship. After I was in practice the technique of leaving a stent was developed. It was relatively easy to extrapolate adding the stent placement as part of the procedure since I was familiar with the basic larger part of the procedure. Other procedures (Kyphoplasties for example) entailed putting a large bore needle through the skin of the back into the vertebral body of the spine and injecting medical type cement to stabilize vertebral fracture caused by osteoporosis, accident/trauma or tumor-like metastatic breast or prostate cancer. I was not familiar with this as it had not been invented before I went into practice. I would attend a hands on conference and read about it first. I would then scrub in with another physician that had significant experience and learn the technique. When I felt confident I would do a number of them under the guidance of an experienced physician. When I was ready to go "solo" I would choose an "easy" case. I always explained to the patient that this was the first one that I had done by myself but I felt competent to perform the procedure. Most patients said ok. It was actually rare for a patient to ask my credentials or how many of a certain procedure I had done, but if they did I was always honest with them.
There are some procedures or surgeries that are just more complex or require more skill than others. Robotic prostatectomy is one of them. Most younger urologists learn the technique in training so have done quite a few when they get into practice. Others have someone mentor them. Surgery requires hand eye coordination just like some other endeavors or sports. Some surgeons are just naturally more skilled than others. It is reasonable to ask a surgeon how many they have done and what their complication rate is. Again, it is your body.
Hope that helps.