About 2/3rds of diabetic dog owners say that their dog has ever alerted them to a low blood sugar, and about 1/3 of diabetic dog owners say that their dog has alerted them at least a dozen times.
Last year I tried to look up data on how many blood sugar alert dogs are working in the United States (or anywhere) and I was unable to find that data. I was not even able to find data on exactly how many service dogs of any sort work in the United States (although I've seen a guesstimate of 20,000). I have met one person who had a service dog prior to developing diabetes whose dog alerts her to low blood sugar, and like most of us who read online, I have read of many stories of both pets (dogs and cats) and trained dogs alerting to high and low blood sugar. I have also read about dogs that were sold as having been trained, but which did not alert to high or low blood sugar.
It is entirely legal to train your own service dog, although not easy, and your dog does have to be well trained in order to be allowed to go everywhere with you (states differ in laws about service dogs in training).
Currently there are numerous programs that offer dogs to diabetics for blood sugar alerts; a google search for diabetes alert dogs brings up 12 different companies offering dogs to diabetics in the first thirty hits.