How many risks are there with breast augmentation?

Worry wart or not, these and many others are wise questions to have answered by an experienced, board-certified (American Board of Plastic Surgery) plastic surgeon who does more than a few breast enlargements per year. I have been in practice for over 20 years, do hundreds of breast surgeries per year, and I still have some patients who request or require re-operation. Fortunately, very few, but my perfection badge has not yet arrived in the mail! To answer your posted questions briefly:
1) The main reason for re-operation is size change, so choosing properly is important. There are lots of ways for doing this--use as many as possible, but also listen to your plastic surgeon. See my article "What is the Right Breast Implant Size for You?" for more detailed information. Other reasons for re-operation are bleeding, infection (requires removal of implants in most cases), malposition, and capsular contracture. My personal re-operation rate is less than 5% for all non-size-related reasons.
2) Capsular contracture is caused mainly by bleeding, or bacteria, so surgical technique is critical. Good surgeons have capsular contracture rates less than 5%.Surgeons whose patients have significant post-operative bruising, swelling, and pain, or who routinely use a drain for breast augmentation cases, tend to have much higher capsular contracture rates. National CC rates average around 10%.
3) Breast pain is mild after surgery and virtually always goes away completely with time. Contracture can cause pain in addition to firmness--that's why it is best to avoid this in the first place. See #2 above. Chronic breast pain (from any cause) is rare! Numbness (some degree) is common, but usually not bothersome (see #4 below).
4) The implant has to be in a space within your body, so some sensory nerves are unavoidably cut, regardless of surgeon, incision placement, or care during surgery. Skin numbness id different from nipple numbness; most skin sensation recovers over time, but 5-10% of women may have permanent loss of nipple sensation. If a breast lift is also performed, the rate of nipple sensation loss is around 15%. When numbness occurs, this does not affect the ability to breast feed.
5) Saline implants can eventually leak and deflate, more common with textured implants, and more common in implants that are under-filled. The fluid is IV saline so this is not a problem, but the deflated implant must be replaced via surgery. Free implant, sometimes free surgery, but re-operation required. National saline deflation rates are around 4-9%. (One of the reasons I'm not a fan of saline implants--and we used thousands between 1991 and 2006, courtesy of the FDA restrictions. Rippling--despite proper overfill--is another concern.).
6) The present generation of silicone gel implants (by both manufacturers) are cohesive and cannot deflate or leak, and take an extraordinary force (or manufacturer defect--very rare--or surgeon damage) to cause rupture. MRI is incorrect 21% of the time (FDA data), and with these newest implants is unnecessary, IMHO.
7) See #6 above. Older silicone implants had a more sticky silicone gel filling (less crosslinking and therefore not cohesive), and could rupture and cause inflammation or capsular contracture requiring surgery to remove the capsule and replace the implant(s). Use only the newest generation of silicone gel implants. See #6 above! BTW, you already have microscopic molecules of silicone in your body from needle lubricant on any shot you have received (like tetanus booster, immunizations, etc.), or intravenous lines. Less silicone will come off your breast implants than you already have in your system. For more information about this, read my article "Are Silicone Breast Implants Safe?"
I have written an 27-page informational article that I give to my breast implant patients. Many more questions are answered; but nothing beats asking a qualified, experienced plastic surgeon! Best wishes! Dr. Tholen

Hi there and thank you for the question
The complications are certainly in the realm of a top 10 list of the most frequent implant based issues post operatively. Now these certainly dont occur often but they are definitely worth mentioning and involved in something called “informed consent”. 
Now there are other complications that can occur but these are even more infrequent and if you are looking for a most extensive exhaustive list then perhaps a repeat consultation or assessment with your surgeon is necessary. 
Best
Waqqas Jalil
MD FRCSC

Thank you for your question.  For the most part, breast augmentation surgery is very safe.  In fact, it is one of the safest surgeries we perform.  However, there are indeed potential complications ranging from infection, to undesired cosmetic outcome, implant rupture, capsular contracture, bottoming out, implant malposition, etc.  Many of these can be found at the link attached or covered in the video attached.