Is it possible for humans to have more than 2 kidneys and are there any health risks if they do?
Yes it is entirely possible. I know this because I have three kidneys: two on the right and one on the left. It is a genetic abnormality in my family and my mother, grandmother, and great grandmother all have it too. There are not really increased health risks associated with it, but I have had kidney and bladder problems for most of my life because of the anatomy. Because one of the kidneys is situated above the other the ureter (tube that goes to the bladder and carries urine from the kidney to the bladder....not urethra that caries urine out of the body) is/was at an angle that allowed for "reflux" of the urine from the bladder back up and into the kidney where it was stagninant (without movement). This caused a few serious infections, one with systemic toxicity and near renal failure. In additon, I had many many UTIs (urinary tract infections). Luckily I got in touch with a good urologist who did a surgery called a ureter reimplantation. They took the ureter and cut it out of it's spot on the bladder and put it in a different spot. This was at age 12, I am now 22 and have not had problems since. No other members of my family have had problems with their extra kidney, some increased UTIs in childhood but not like mine. Also, I should have mentioned my kidney does not work, but we think that my mom's and grandmother's do (so they could donate to someone in need). Hope this helps, and to the person who said no this couldn't happen, don't be so quick to say no (or in your case know) when you don't know.
How long can a person go without urinating?
I can usually sleep the night anywhere from 10 - 15 hours without peeing but any longer might start to push my limits. Urinating has nothing to do with prostate cancer and the pee issue will not cause cancer of the prostate.