Our last cocker, Freddie, only had one, so it was removed (with the other one). Sorry, it's too long ago to remember how old he was (mum organised it too).
Henry also only has one, although I feel him up (

) every day to see if I can feel the other. I'm sure I can feel a second one under the skin (the first hasn't popped down yet), but the vet couldn't when he saw him last week. They may drop down at any time, apparently. He's now 19 weeks.
I plan to get Henry neutered when he's 8-12 months. Obviously the operation is easier if the balls have both descended. If not, then the vet has to dig around to find the other. Sometimes they are easy to find, sometimes take a bit longer.
There are various studies that show that retained testicles are more susceptible to cancer, and also that just having testicles at all increases the cancer risk. Other studies show there's no more risk. Most vets seem to push it, possibly because they want to see fewer unwanted puppies, or possibly because they make money out of it (although I doubt that this is the case, as they'd make a lot more from cancer treatment).
I'd take the advice on here about castration with a pinch of salt though, as it is such a personal thing (and there's a study to suit everyone's point of view) - whether you get your dog neutered seems to be a very big bone of contention on here - some people recommend it, some people don't. I think most people would agree that you shouldn't leave an undescended testicle though, whatever you decide to do with the "good" one!