According to the DEA doctors must be certified to prescribe Suboxone and they are pretty strict about it's controlled use for opiod dependance. There are more than just a handful of doctors, it might be the case that in her area there is only a handful but where I live I found at least 100 different doctors. They have an actual website that you can find a doctor in your area who is legally allowed to prescribe it. The mfg website indicates that it is used for:
Suppress symptoms of opioid withdrawal
Decrease cravings for opioids
Reduce illicit opioid use
Block the effects of other opioids
Help patients stay in treatment
You can't really compare it to oxy or fentanyl because it's not used in the same way. It's primary reason is for depedance not pain management. If one were to compare it to anything it would be compared to Methadone as it's primary use is for the treatment of herion addiction.
I'm not sure why or if there is really a restriction on the amount of individuals they can treat at one time, the DEA does not specify that, it only specifies that they must be certified in order to prescribe it.