I suspect that a knee brace is overkill for the vast majority of people on the Camino. However, as someone who no longer has ANY ligaments in my knee (due to a Camino accident and 3 surgical procedures;
http://www.caminodesantiago.me/community/threads/health-insurance.6068/), I used this on my left knee while walking last year:
https://www.shockdoctor.com/ultra-knee-support-with-bilateral-hinges. I tried several other ones before settling on the top end Shock Doctor one; it is far and away the best/most comfortable/most supportive I found. If you
need a brace, consider this one.
I also
strongly recommend that you use a sheath under the brace to prevent chafing. I recommend either Sun Knees in hot weather (
http://shop.pearlizumi.com/product.php?mode=view&pc_id=125&product_id=1794903) or any cycling knee warmer in cooler weather.
If you have knee problems of any sort, I also strongly recommend using trekking poles (especially
http://www.pacerpole.com). If used properly (something I saw less than 10% of users doing on the Camino), they take a
huge load off of the knees, especially on descents. If used improperly, they are practically useless.
I am happy to answer any questions about my own experience with walking the Camino with a
very bad knee. At one point, I told my wife that my knee was the
only part of my body that did not hurt.